Keeping an account of your costs of investments and their performance relative to other options will help identify personal strengths and weaknesses. Often just moving to strengths will increase profitability, while recognizing weakness will at least help you recognize what trades to avoid if you can't take more time putting them together.
Taking My Own Medicine
Personally, I can't stand stuff like Rhox Faithmender (the subject of a recent QS email). Here I see a cheap card getting eaten up in an event following an increase in red play. It is interesting tech and can swing a life gain race between competing Thragtusks in a player's favor. It's also relatively easy to splash.
So what's my problem? Well, it isn't a card that presses to victory. I am reminded of really bad legacy Dredge sideboards that take a reactive approach to decks instead of offering different sacrifice outlets for the pilot. Beating mid-range by playing a mid-range card is great if your deck is already a mid-range deck. More aggressive decks don't need this and combo doesn't (I say combo like that's a thing). Right now Rhoxy is $3.50 with shipping on TCG. The next two weeks are a great time to unload him for value. If you can pick him up at bulk, go for it but I am going to avoid this thing at $1 or more.
Let's mark that : Rhox Faithmender, sell recomendation on December 3rd TCG price $3.49
Meanwhile, in the world of MtG cards playable in any format, Cavern of Souls is available for $19.15 on TCG. I don't think Cavern gets under $18 until it rotates out. After the typical time out of standard this piece of real estate likely settles around $20. Every tribal deck wants counter protection for its creatures.
Big old BUY recomendation on Cavern of Souls, today at $19.15 TCG
Bonfire of the Damned is around $23. Have you tried picking these up? My local store doesn't have them and the players that are holding are all sharks. Maybe <$25 is too low for this miracle. Afterall, who is going to play this as a single copy? Bonfire is a card that remains under-appreciated across other formats, but it will still make its way into more powerful formats once people realize setting up miracles isn't that hard and this thing isn't getting less one-sided. That said, it probably settles <$10.
Boros is coming. Buy <$25, up now on TCG for 23.
Sphinx's Revelation is a good card. It's got blue for FoW. It draws X at instant speed. It's mythic! I played two copies in a deck and it started talking to me in game, asking me why i wouldn't play at least one more. Unfortunately this card is clunky. Blue Sun's Zenith is a better card most of the time (it can be redirected, which sucks) and post-rotation Zenith is <$0.50. Until this Mythic gets back to $8 I won't be adding. In fact...
I have 2 Sphinx's Revelation for sale get yours today. TCG says they are worth at least $20. You know you want to prove me double-wrong. Lets swap out my Revelations for your Bonfires.
The Oft Forgotten
Ajani, Caller of the Pride. Go look at the spread of prices for this guy on TCG. It's nearly ten to just over twenty dollars. Looks like a lot of people having a hard time finding a price for the Caller of Prides. I think 10 bucks is a great price for a 3cc walker that is getting Boros to work with in Gatecrash.
Going a Little Deeper
Five is good enough on big stuff. Now i'm going total speculation on you.
How much is the FNM Lingering Souls worth? 3x as much as an uncommon with the same name from Dark Ascension at my local store... but they don't have anymore. Lingering Souls is a good card. I think w/b is a thing with plenty of good cards already and a guild coming to save the day on the way. I love trading for commons and uncomnons. If you want to get something that will see play in legacy and vintage get a pauper game going at your store. Trade pauper staples for O-Rings and Souls.
Tracker's Instinct, Thought Scour, Desperate Ravings, Grisly Salvage.... all these are better than the average. Leverage them into money now as uncommons or bulk rares. Get foils and you'll find buyers on ebay. Right now the foil Charms are outta control hot. I have a playset of Izzet Charms at todays prices, and I preordered them.