Leroux: How much is a few bucks?
If you like challenging platformers, maybe
Project Black Sun ($1.99) would be up your alley? I haven't played it myself yet. Also,
Climb to the Top of the Castle is free (or PWYW).
Home, an indie-horror adventure / experiment in story-telling is $2, too. Some people enjoyed it a lot apparantly (I didn't).
If you like Arkanoid / Breakout, I can recommend
Wizorb ($2.99).
Blocks That Matter is a great puzzle platformer available for $4.99.
If that's still in your price range, there are also two rogue-likes for the same price,
Cardinal Quest and
Dungeons of Dredmor. As for best value for money, I'd recommend Dredmor over Cardinal Quest, the latter is short, more casual and without save function.
And like the others say, you can always get a lot of good games for free.
Here's a neat list for browsing!
Tried Dredmor once but couldn't really get into it but I may try again.