I found a copy of Shadow of the Colossus for only $5 (I think?) at Cash Converters. It was a bit battered and I had to get the shop to run the CD through their disc-fixing machine, but it still had the manual and all the art cards in good condition. I also got The Curse of Monkey Island there for $3.
Apart from that, every time I go into my local GAME, I see a copy of the PC version of Mirror's Edge marked down to $4. I already have a copy of Mirror's Edge, but every time I see it I have an irrational impulse to purchase it, because it's such a bargain! (In case you can't tell, I really like Mirror's Edge. Despite all its flaws.)