Am I late for the 'underpriced cards' train?
Fierce Empath (0.50)
Should be in nearly every green EDH deck.... Fuck, casual elves should probably be running a copy or two, just because it's so flexible. Never underestimate the power of a tutor that leaves behind a body -- the creature you grab is likely going to be able to make use of it. If not, it carries a sword later on.
Evacuation//Cyclonic rift ($1)
Targeted removal won't kill anything with hexproof.
Boardwipe won't kill indestructible creatures
Mass damage won't kill creatures with "protection from X"
These spells 'kill' everything, in the sense that they'll get them the fuck off the board at instant speed.
Archaeomancer (0.10)
Chains with evacuation. Wins games in limited by giving you backbreaking card advantage. Tied with academy raiders and dragon hatchling for the most times I've seen a good card go unacceptably late in limited .
Azorius Guildmage (0.10)
This guy hitting the board early enough in a U/W deck with enough lockdown = gg. Stifle on a stick, encourages interaction between expensive decks on the same table, allowing the 'fair' decks to catch up. Should be in every EDH deck capable of running both colors.
Hellkite Tyrant ($1)
For most competitive constructed play, this guy is pretty bad. He's... sideboard card #15 in your bad sneak attack deck in a meta that does not exist.
EDH and casual, whoaman.
Permanent and crippling card advantage from one hit. Find a way to give him haste and you'll find he can create situations it's impossible to recover from.
It's almost a shame he didn't have it by default, but then he'd easily be red's equivalent to Primeval Titan in the format -- comes into play, takes a few 'lands', gets pathed, everyone's mad at you. This one has a slightly implausible win condition attached, too!
The 10-card megacycle of lands from Innistrad block ($0.10-$1)
To start, Moorland Haunt. A free 1/1 spirit when you have some leftover mana is a worthwhile investment. Saw heavy standard play back for a while. Is a land. Is 0.10.
are you freakin' kidding me?
What is white best at? Small creatures and mass pump. This gives so much reach to tempo decks it's absurd. Expect it to come back in modern at some point when W/x hatebears becomes a tad more viable and/or is encouraged to play blue.
Kessig Wolf run is fireball every turn, but a land. Innistrad is in fairly short supply compared to RTR block. $1 is nothing. Buy every copy you can right now.
Gavony Township has already proven itself to be a very potent card in melira-pod, playing a bigger role in the deck than... well, one of its namesakes, at least (melira). Won't discuss it much, but I will mention that it's a SEVENTY FIVE CENT CARD. When arguably format-changing rare lands cost under a dollar, you need to be buying them.
Nephalia Drownyard is a thing. It acted as a kill card in standard grixis control decks for a while, and now it's just sitting at 0.25, waiting to be picked up in another format.
It won't be, but it's still very fun to self-mill with in casual.
Rounding out the allied color lands is whatever the RB one is. Don't buy it or play it in any deck ever.
Slayers' Stronghold is very powerful. Granting haste to late-game drops is yet another way to make weenie rush-style decks relevant. Saw some play in a World Champs deck, to moderate success. Otherwise, has flown under the radar, as an occasional 1-of. Obscenely powerful in EDH, but in unpopular colors -- still, I'd run it over Hall of the Bandit lord.
Alchemist's Refuge should be considered an EDH staple at this point. It's in the two most popular colors and makes it so you can drop your sorcery-speed bombs at end of turn. Hell, it lets you wipe the board right before your turn starts.
Vault of the Archangel sees a tiny bit of play in competitive modern, oddly enough. Lingering Souls is best friends with this fucker, and despite the activation costing more than an average deck's most expensive spell, it manages to get work done. You'd assume the high cost and lack of ability to generate colored mana would force it out of the token decks it wants to be in, and you'd be mostly right, but it's still a powerful enough card to justify inclusion in some decks. It's still probably the third most backbreaking effect out of any of these lands, next to township and wolfrun.
Grim Backwoods is a weird enabler for EDH. Ignore it in 60-card unless you desperately need sac outlets and cards. Maybe in some Rude Awakening deck.
Desolate Lighthouse sees fringe play in modern (1-of in the format's most popular deck), and is generally very good. I tend to build EDH decks that have more to do with their mana, and still have a hard time keeping this out.
For more competitive cards that're underpriced right now (with the exception of the VERY playable lands above)
Well, I was working on a post that included Horizon Canopy, but I guess it just jumped to $30. Damn, I hadn't even bought mine yet.
Some others:
Omniscience, which is playing a role in Legacy right now, only recently broke the $7 mark. I bought in at $3, but it's still a unique (and obscenely powerful) effect. Core set mythic, too. It has a LOT of room to go up.
Dismember is one of the five most played spells in modern. It is a small set uncommon, yet is (bafflingly enough) under a dollar. I'm sitting on half a longbox full (500), and will likely buy more. Yes, it was in a number of event decks. Insignificant compared to the print run of a normal uncommon.
Deathrite Shaman is the third most played card in modern. They're in large quantities right now due to the influx of new players post-RTR, but you should still be aggressively getting these out of binders. Future $40 card.