Well, I don't know about the availability of these games in the West, but at least in Japan, the following three are among the most common/cheapest PlayStation 3 games around...
- Jojo's Bizarre Adventure: All Star Battle
- Afrika
- King of Fighters XII
Dragon's Crown
(I'm actually astonished to hear that
Afrika is considered a rare game... It can be found for litereally a few cents here)
Of the games I either own or have on my radar, I'd guess that
Chōjikūyōsai Macross: Ai Oboeteimasuka - Hybrid Pack (Shokai Genteiban 30th Anniversary Box),
Ketsui: Kizunajigoku-tachi,
Dungeons & Dragons: Mystara Eiyuu Senki [Chronicles of Mystara] and
El Chavo Kart [Chaves Kart] are some games prone to become rarer and more expensive as time goes by...