Someone already said it a little higher, but the Orange Box is classic for the price. That will last you a good long while.
If you want to splurge and actually pay for a game that will last a long time, go with Fallout 3 if you haven't already. Instead of looking for cheap games, go for quality. You might buy a 10 dollar game and realize it's crap, then spend another 15 on another 'meh' game and finally end up downloading an 800 MS point game...instead, get a high quality game. Gears is great single player action with a robust multiplayer experience, same with CoD games (get Modern Warfare over World at War). Bethesda is amazing with their games and they last forever...