Imo, the only 'under valued' card is the black delve legend. Tasigur. the Golden Fang. Japanese were buying out starcity 40 at a time till they raised it another $1. The dude is basically a 4/5 for 1 or 2 mana in the right shell (t1: thoughtseize t2: satyr wayfinder, t3 Tasigur for 2 mana.) that also basically draws you a card for 4 mana repeatedly. If you are delving things you don't want, and leaving a land or two as the others, your opponent ends up giving you the card you want. Plus, it fuels more delve shenanigans later.
He's very interesting, and since we're already on that path with sultai reanimator decks, does not seem far fetched to have a deck that synergizes very well with him.
Other then that, 80% cards are all overvalued at release. Soulfire and Monastery Mentor are both going to drop to like $15-20 range about a month after release. Everyone knows this, but people need their playsets early on so the price is inflated. It's still cheaper to buy singles at these prices then opening boxes hoping to hit the cards you need. Trust me, I'm doing pre-sales on ebay (pinnaclegamestore is my user ID if anyone is interested) and the profit margins are pretty low. If I sell every card in the set at presale prices, including commons, I'm clearing maybe 10-15% after ebay fees versus the wholesale cost of a box (which is now almost $80 from the cheapest distributor, prices went up for new year).
If I don't sell more of the common/uncommon playsets, I end up breaking even or losing money just selling the rares and mythics. Hoping to get lucky with some foils, and that fetches end up being 3 per box rather then 1.5, but that is unlikely.