Best Bang for the buck is inevitably The Orange Box, why you say? Its around 20$ - 30$ new and carrys 5 excelent fps games. Read some reviews about the orange box sure it very badly rated on the ps3 but I have it and so all my friends and its the best game we couldve got.
First if you havent plaed half life 2 already seriously you are missing out and the orange box has it and its sequels episode 1 and episode 2 after that you are left with portal which was an amazing game again read some reviews about the orange box or portal and you will know what I mean and finally the 5th game is team fortress 2 which is a blast to play and has a very loyal comunity on the ps3. You will probably read bad reviews for the ps3 like some bugs but the game is totally playable entirely and its very fun.
Sure TF2 doesnt carry all the updates the pc version has but I own the pc version and I still like a lot my ps3 version because its where I play with my friends on the pc I dont have any real friends to play with but its amazing, you dont need friends to play this kind of game its lots of fun everywhere.
I downloaded first half life 1 since I didnt play it and then started with half life 2 it was the best and its the best bang for the buck just check out the reviews and people wont let me lie.
For other great shooters I love on the ps3 and again please read the reviews.
Resistance Fall of Man, this game is really very good and anyone with a ps3 should play it, multiplayer is dead but single player its just excelent.
And lastly for me
Call of duty 4: Modern Warfare, sure COD5 is already out but I didnt like it as much because it went back to wooden machineguns and everything is now old and underpowered compared to the great weapons cod4 has and its like 30 bucks now cod4.
The 3 games im recommending are less than $30 bucks and are THE BEST FPS on the ps3. My next purchase will be resistance 2. Be happy my friend and read reviews about the 3 games Im recommending.