If you want a cheap game you can buy fallout 1 &2 in a bundle.
Great games. Graphics arn't the best, but it has great open based gameplay. You can be evil, or good, but the game doesn't force anything upon you. It doesn't even force you to go anywhere, although it encourages you with a 150 day (game time) time limit, but that is more than enough time.
If you want multiplayer, try fallout tactics. Better graphics, still an older game, but it has multiplayer, and vehicles.
I'm a big fallout fan.