Im from the UK so prices may vary as I know games often don't convert properly on price
The Witcher 2 - Very very good RPG you can play this through a few times and follow different paths each time, visually pretty awesome too
Red orchestra 2 - The online element is big on this but there is a solid single player campaign. It's a decent WW2 based shooter and pretty playable
The Half Life games - These are classics and worth a go.
BTW - I decided to start with Half Life 2 - orange box(has 5 games)