Modern bliss
Bioshock: Only paid $20! Damn good deal!
Grand theft auto: I think i paid $40 for this, but full price would have been worth it!
Starcraft II: I'm going to be farting around with this game for years!
Mass Effect 1: Another $20 deal! Practically theft ^_^
Mass Effect 2: Full price, totally worth it!
Obliv: Can't remember buying price, would have paid full
Fallout 3: Full price and worth it.
oldschool nostalgia
Zelda-ocarina of time: If i remember right, this game released for around $70. Still worth it!
Soul Calibur 2: Always worth it! If you don't have it, get it. Even if you own SoulCal 4, get #2 anyways.
Final Fantasy VII: We had multiple copies and multiple Sonys for this game >_<.
Final Fantasy Tactics: Never sold for much, and i don't know why. If you have a PSP, get it.
You-do-the-math section
Disgaea 2: $30 and a 130 hours on one file.
Tales of Symphonia: $18 and 170 hours on one file. Best. Deal. Ever.
Tales of the Abyss: $30, 130 hours on one file.
Pokemon Plat: $35 (full price), 95+ hours on one file. Loves me some pokemonz!
Final Fantasy Tactics A2: $35 (full price), 98:25:31 on one file.
Honorable mentions
Nehrim: Obliv full conversion for free. can't get much better than that!
Torchlight: $20 game, is decent. I've had better deals, but those required waiting 3 years >_<