The games with the best value that I've got at the moment are:
World Of Goo - 1500 points, a physics based puzzle game. Great style, great gameplay and better than a lot of retail games.
Art Style: Orbient - 600 points, I play this whenever I have nothing to do for 20 minutes.
Tiki Towers - 500 points, a World Of Goo clone. It's nowhere near the style or as polished but it's cheap, lasts about 7 hours (plus replaying if you want to better your scores) and it has monkeys in it.