Fallout 3 with all the DLC. I've racked up like 200+ hours in it.

one of my plans is actually to sell my copy of Fallout 3 and get the GOTY edition that has all the DLC on it, however, the GOTY edition comes out in the holiday season, besides, I think only the first DLC pack has come out for the ps3 so far

If you like SRPG or Anime Disgaea 3 for sure, the game has a very very extensive amount of content that demands multiple playthrough at the very least. Other game with longetivity include Dynasty Warrior 6 Empire (very very repetitive but if you dig button mashing then its got enough content to last a couple of months easy), also look into SFIV and Killzone etc etc if you enjoy online play and ranking.

Eternal Sonata.
lovely lovely game, though i don't know if you already have it or if its long enough

Valkyria Chronicles
also lovely game

Disgaea 3
not for everyone

Lost Odyssey
Amazing old school RPG and old school is the new cool

Oblivion
better than Fallout IMO

I was going to suggest Wii Sports but then I saw your 80 dollar budget, bleh!

well MGS4 can be played a million times and for multiplayer killzone 2 will last you several motnhs at least

he has CoD4 already. I know they are different games, but he has an online shooter already is where I'm getting at.

how about far cry 2 you guys? It seems like it's got a lot of open world gameplay

Left 4 Dead has great online.

there's only one game - Sacred 2. this game is friggin HUGE, it's gigantic - even if you would only go straight from one main quest to another it would probably took weeks before you would finish it, but you can always just move around and slice everyhing you see and there's hundreds of sidequests.