All of the games listed below are at least somewhat highly rated but still cheap games. You should easily be able to find any of these under 15 dollars somewhere or another. I'd recommend Amazon
as a place to start looking.
Dragon Quest Heroes: Rocket Slime

Rocket Slime is a surprisingly fun little spin off the Dragon Quest series, and you can also keep in mind that a similar Final Fantasy spin off called “Chocobo Tales” can be found for a low price at times too. Both games have very simple but entertaining gameplay that make them worth playing. It's like a fresh breath after a long satisfying RPG, plus you can probably even find it for under ten dollars if you look enough.Sonic Rush

A lot of recent Sonic games haven't been so good, but Sonic Rush is definitely one of the better ones. Funny how it's also one of the cheaper ones, so it makes it kind of a win-win situation. In Sonic Rush you'll be playing in the classic 2D style of Sonic and the “catch” of the game is that it uses the duel screens, which some liked and some hated. Either way, if you have never tried it, it is definitely worth the money.Viewtiful Joe: Double Trouble

This also happens to be one of the better Viewtiful Joe games out there (of course nothing has ever topped the original). It has surprisingly nice graphics for the DS and tries to stay as true as it can to the originals gameplay while adding some interesting DS gameplay that most critics have liked.
Trace Memory

Trace Memory is one of the more prominent “Point and Click” games out for the DS. It's a very pretty adventure game, and as long as you keep in mind it is mostly about solving simple puzzles and reading dialogue, then this game is worth the money.Feel the Magic XY/XX

This is basically a hilarious collection of mini games where you play them all just to charm a girl. It's a very fun game to play, but it is pretty short as there are not many mini games. Luckily you can sometime find this one for around five dollars pretty easily.Elite Beat Agents

It's amazing that this one can be so cheap, as it is one of the best games you could possibly buy for the DS. Elite Beat Agents is also just one of the better music/beat games out there is general, so don't pass if up if you've had any interest in the genre before!Lost Magic

This may not be the best RTS for the DS, but it sure is the cheapest. It holds up well as a 30 dollar game nonetheless and was one of the earlier games to use the DS internet connection. If you like real time strategy games, then this is probably worth the price it has now.