Well, I got my PS3 specifically to play Metal Gear Solid 4, so I know what you mean about dropping cash for a single game and not really feeling justified in the purchase. However, if you like games other than RPGs (personally I have always loved them but FF13 was not my cup of tea at all) there are a lot of really great games. It depends on what you're into.
Little Big Planet is tons of fun.
Uncharted and Uncharted 2 are amazing - 3 is good but not up to the bar they set with 2.
Batman Arkham City is just about THE perfect game.
Any of the Lego games are so much fun, cheap, damn cute and funny. (I recommend starting with Lego Star Wars the Complete Saga, especially if you know the movies.) Heavy Rain is a beautiful game, but can be tricky control-wise.
Resident Evil 5 is good.
Dead Space and Dead Space 2 are phenomenal.
Red Dead Redemption is fantastic (GTA on horseback), and Saints Row the Third is effin brilliant (GTA on meth/coke/ecstacy).
Any of these games can be bought at a GameStop or EB or what have you, used for cheap.
I don't play FPS so I can't comment on any of the COD or MW games. And the upside to PS3 for me is that they have a great back catalog of classic PS games, and eventually will offer more PS2 games too.