If you head to the PlayStation Web store, you can actually click on the link for games, then all games, and then for the fixiest of fixes you can link yourself to games within a certain price value. Of those are games that are free, those that are $5-$10, and if you're a little more curious games that are between $10 and $20.
Here's a link for you to check out games that fall within your determined price range, including Ms. Germinator, Superfrog HD, and Divekick. Personally though, I'd recommend PixelJunk Monsters. It's one of most addictive games you can find on the PlayStation Network.