Actually, I've been finding Pac N Roll to be fun as hell for some reason. My friend had it, and lent it to me. Also, Ridge Race DS is fun, but a little difficult to control (It's like tryin to play a racing game on PC without a gamepad or wheel. There are touch screen analog options, but they suck imho). It should also be dirt cheap.
Also, as other have said, can't go wrong with:
Mario Kart
Meteos
NSMB
Tetris DS
Metroid Pinball (I loved it, but you need to like pinball prolly)
Castlevania
Advance Wars
Try before you buy: (I thought I'd love these, but was very dissappointed)
Yoshi Touch and Go
Metroid Prime Hunters (where are the Metroid adventure elements of the GCN Prime series, this is a friggin FPS! If it wasn't Metroid, I'd prolly have liked it, but I expect more from that series)
And don't forget the ability to play awesome GBA games:
Castlevania (all 3)
Final Fantasy Tactics Advance
Final Fantasy I & II Dawn of Souls
Final Fantasy IV Advance
Mario and Donkey Kong
All the Mario ports
Metroid Fusion and Zero Mission
Golden Sun 1 and 2
Astro Boy (no, hear me out. Do you like old school side scrolling games like Megaman, Gunstar Heroes, or Contra? If yes, buy this game. If no, buy this game and find out what you are missing!)
...as you can see, I play an exorbitant amount of portable games (gotta love those train rides), and play tons of them. There are far more out there, but these are the ones off the top of my head. The first 2 DS games I mentioned should be pretty easy to find cheap. For the other DS games, to stay cheap, I suggest trading, renting, or ebay. You are right, DS games rarely drop in price, sadly. Luckily, if you look into the GBA games I mentioned, you should be able to find a good portion of them fairly cheap, since GBA is an "old" system.