What if I told you he'd been used in successful Legacy/Vintage decks because the Portal wording has been eratta'ed to say "return two target lands to your hand"? Does that change your guess any? Most Eternal players will know that he's worth a fair bit more than the price of a couple White Castle hamburgers, but the rest of you will be absolutely shocked at this little guy's price.
He's worth over $40. Why? Well, he's a 4-power flying 3-drop in Blue. His drawback, with the errata, is so far from relevant that it's funny. If you stack his trigger, targeting 2 Islands and then use said Islands to pay the alternate cost of Gush...well, let's just say that if Mulldrifter is good, Sea Drake + Gush is better.
The lesson here is not that you should play more Eternal - although you should because it will teach you lessons that are applicable in every format you'll ever play. The lesson is that there are extremely valuable cards that most people don't even know exists. Ever heard of Guru lands? They're one of the earliest promo sets of basic lands with gorgeous art by Terese Nielsen. You can ID them by a leafed gold expansion symbol. They're worth anywhere between 20 and 60 dollars depending on which one you can find.
These are both examples of cards I've found in unsorted, bulk rare, or junk boxes. Gaining incremental advantage here and there via trading is great, but sometimes you just need to hit the mother lode and find a hundred dollars worth of cards for a dollar. You know those big 5-row, 5000 count boxes filled with cards at your local game store? I can almost assure you that unless someone like me has scoured the box twice, you'll find something of value in there. Did you know that Priest of Titania, an Urza's Saga COMMON, is priced at 4 dollars? Bet you can find those for a nickel or so in most stores.
If the first rule of thumb is that there's ALWAYS a mistake in bulk boxes, the second rule of thumb is that you're always going to miss something the first time through. I know I missed a 20 dollar Portal rare in a bulk box (Cruel Bargain) because I got lazy and didn't price check it on my handheld device. So, don't be lazy, check your prices on the fly, and always dig twice. I believe that someone came in the next day and bought it, because when I came in the Bargain was gone. That leads me to your third rule of thumb: keep your mouth shut about your sources! You did the hard work to find a good stash of cards, so why tell everyone about it? Loose lips sink ships, and they also ruin potential goldmines for Magic bargains.
The same thing applies to buying collections. Many people selling out of the game will give you a huge discount on their cards just to make a quick sale, so follow the advice in the Trader's Guide and always have some money around to buy out a collection. Be smart and realize that 99.5% of the stuff in someone's collection isn't going to be worth the cardboard it occupies, but you can usually find deals.
So, head to your local FNM tomorrow and ask your store if they have a Dollar Rare box, a box of unsorted older cards that you could buy at a bulk rate, or any other stuff like that. Leave a comment if you find any bargains, and keep an eye on the front page for Part 2 of A Trader's Guide.