Bear in mind that Magic's prices are dominated by Standard competitive play, and that my analysis comes with exactly that bias. Commander / EDH can also drive prices, but not to the extent that Standard can. Legacy, Vintage and Extended are all healthy, enjoyable formats and all that good stuff, but Standard is the format that makes the trading game work. These cards all have their merits, and not every card was designed to be good in competitive Magic. Bearing that in mind, my only goal is to tell you where to put your money, not how to play around the kitchen table.
Batterskull - $25
The good folks at Wizards of the Coast decided that Stoneforge Mystic didn't have enough friends, so they printed Batterskull. I have absolutely no hope of ever defeating this card in a Stoneforge Mystic deck, as the ability to flash in a 4/4 lifelinker with Vigilance at the end of the turn is absurd. In case you had fantasies of actually killing the damned thing, keep dreaming. It bounces for a meager 3 mana, and when you replay it (with Mystic), guess what? You get another free dude. Bear in mind that, even though the Germ token is 0/0, equipping it with a Sword and then bouncing the Batterskull is even more card advantage! This card is pretty absurd, and its only saving grace is that Stoneforge Mystic is leaving Standard in the fall. I don't plan on touching Standard until then. I'm loathe to say it, but it's high price tag seems justified. Even decks like the Grand Architect deck can abuse it. Hope you don't play red.
Elesh Norn, Grand Cenobite - $6
By the time he hits the board, you're dead. He's irrelevant to most formats, including Commander, since -2/-2 shouldn't really scare any EDH player on turn 7. Get what you can from this bulk Mythic, since there's no way it'll live up to its modest presale price.
Etched Monstrosity - $3
There's good Mythics, there's bad Mythics, and then there's value bets. Etched Monstrosity falls into the third category. A $3 Mythic is hard to pass up, especially when the payoff for correct usage is a handful of cards and a 10/10. Yes, it fails the Doom Blade test, but everyone's running Go For The Throat anyway. It'll likely not see play in Standard or any other competitive format, but this will find a home in Commander. If your local players are all Spikes and these are virtually free, buy in as a value bet and hope to get paid off by EDHers.
Jin-Gitaxias, Core Augur - $5
I may eat my words, but Jin-Gitaxias is almost Emrakul-level good. He doesn't ruin your opponent's board in the same way Emrakul does, but considering he has Flash and costs considerably less than Flying Spaghetti Old God, I'll make another value bet here. Everything I've said about Etched Monstrosity is true here, and then some. It will be tough to make him work in competitive formats, and Legacy decks have better Reanimator targets, but the prospect of keeping your opponent in topdeck mode for the rest of the game is one well worth mentioning. It's not a sure bet by a long shot, but cheap Mythic Rares with unique effects always make my watch list.
Karn, Liberated - $50
If you preorder this card, you are not allowed to read Quiet Speculation anymore. Considering that Despise was printed in the same set, which can strip the 7-drop out of your hand at any point in the game, I would be more than a little surprised if he saw widespread constructed play. Because I know I'm going to get some serious guff for calling this card "awful", here's my take. Let's assume he hits play unmolested, which is far from a sure thing. Suspending disbelief for a moment, we get a few options.
A) Double Vindicate for 7 mana. -3, -3. I love me some Vindicate, even when it's called Maelstrom Pulse and doesn't hit lands, but the appeal is that you can, you know, cast the spell. 7 mana to blow up 2 permanents over 2 turns does not set my heart aflutter.
B) Mix n' Match: +4, -3, +4, -3, etc. This is probably the "right" way to play Karn, since you'll be net positive on loyalty counters and doing something relevant each turn. Getting Karn up to 10 loyalty on the first turn is the only correct play in my mind, so alternating between ruining their hand and their board seems fine for 7 mana. When compared to other 7-drops, I remain underwhelmed.
C) Go For The Gold: +4, +4, -14. If you hit his ultimate, you will almost assuredly win the game after you reboot. That is good, because if you can manage to stick a 7 drop and protect him for a few turns, you had jolly well better win. Gideon Jura shines in protecting his fellow Planeswalkers, as does Jace, the Mind Sculptor. If your game plan is to overload your opponent's Planeswalker-killing ability with Birds and Swords and MOAR PLANESWALKERS, then, best of luck to you. There seem to be better top-end options in control decks than Karn, and the printing of Despise really does hurt his playability.
Even if Karn breaks camp with the team, he won't be getting a lot of play time. No deck will run a full playset, which will keep the price down. Don't expect his price to drop any time soon, as most dealers have no incentive to price him reasonably. I can't fault them for this, but I can advise traders to wait on acquiring their copies if they feel they need to do so in the first place. Every Commander player will need one, so expect foil copies to sell for more than your automobile and first-born combined. Japanese Foils will require blood sacrifice. Have fun, kids. If you'd like to be liberated of your money, go ahead and buy some today, but if you want to, you know, not waste money, I'd wait a while.
Phyrexian Obliterator - $25
A $25, 4-drop, 5/5 that fails the Go For The Throat test? Where do I sign up? Actually, instead of signing anything, just give me the pen so I can slowly disembowel myself instead of ever buying into this awful card. Yes, it makes for a lovely blocker and it's rather difficult to handle him in combat, but considering how much removal exists in competitive Magic right now, there's no way this will see play. I love the flavor, and I love that my boy Phyrexian Negator got a big upgrade, but this card is a Timmy card all the way. Buying in at $25, the only thing that will be obliterated is your bankroll.
Sheoldred, Whispering One - $5
I liked this card better when it was two cards, not a creature, and didn't cost 7 mana. It's the prerelease promo and that should keep it squarely as a bulk Mythic rare. Not a whole lot to say here. Just worthless.
Sword of War and Peace - $25
Enough already with the swords. I've already given up trying to fathom how a damn bird can hold a sword (let alone two), and now you're giving me more swords to worry about? Enough is enough. This card is nowhere near powerful enough to see play over the absurdly good Sword of Feast and Famine. I've watched players windmill slam down the new SOWP in playtesting, and the slam is often followed by the comment, "I wish this was Sword of Feast and Famine." Apparently, a variable Lightning Helix isn't as good as doubling your mana every turn. In other news, the sky is still blue, water is still (debatably) wet, and the sun rose again this morning. Stay tuned to Quiet Speculation for more news as it breaks.
Urabrask, the Hidden - $8
This is one of the few Mythics I feel is totally underpriced. A 4/4 Haste for 5 isn't all that world-beating, and 5 mana is a lot to ask out of a red deck, but this card is not for the Red Decks you're used to. Big Red, which has been making a showing here and there, can use Urabrask to dramatically turn the tide of any game. Granting speed to your whole starting lineup is a new effect in this format, and a myriad of decks will find a use for it. Kismet'ing your opponent's guys is also new to this Standard format, and with a bit of mana acceleration, Urabrask stops looking so hard-to-cast in red decks. Red suddenly has a midrange game, and alongside beatsticks like Koth of the Hammer and Kuldotha Phoenix*, red decks can move away from the all-in, Boros/Goblins style, beatdown decks and into a slower and more powerful role. It's hard to say that Urabrask, the Hidden will go up tremendously in price, but I'm really, really long on this card and will be putting a very high buy price on him once the set is out.
Vorinclex, Voice of Hunger - $4
Another potentially underpriced Mythic, I can see this Praetor as a 2-of in Primeval Titan decks. It's very difficult to dig out from underneath the lockdown effect, and the amount of mana that Vorinclex can generate is truly absurd. The Praetor cycle creates a beautiful asymmetry in the game, making it one of the most ridiculous and flavorful cycles the game has ever seen. I see why Phyrexia won the war. I don't know how to beat this card once it's on the board alongside Titans, so hope that the RUG-type decks can't hold up in the metagame. I'd call this another value bet, but I'm not sure he'll see more than a little bit of play so don't get too excited and overpay.
Altogether, the Mythic Rares are dominated by the Praetor cycle, which will have a minimal impact on Standard. The star of the set is Batterskull, not Karn Liberated, and a few of the aforementioned Praetors will have a shot at effecting competitive play, but the real power in this set lies outside the Mythic Rares. The prerelease is the best time to dump overhyped cards, but don't expect people to come off of their NPH stuff for a few weeks. Instead, be the disciplined investor who trades away a Karn Liberated for a bunch of Fetch Lands, which are literally never going to be bad.
*Regarding Kuldotha Phoenix, I feel obliged to mention that I bought like 250 of these on Magic Online a few weeks ago because I thought they'd be good some day. I'm not trying to pull a pump and dump here, hence the disclosure, but I honestly believe that the card will be integral in a Big Red deck of some sort, hence the large-scale purchase.