I have no idea where you're shopping, but very few DS games are going to be anywhere near $40 anymore, unless they're very rare or collectable. Go to Amazon and basically every DS game worth playing can be had between $5-$25.
3DS is slightly different, but.....it's brand new, and plenty of deals can be found for below $40.
Chrono Trigger, Final Fantasy 4 (possibly, haven't looked at them since I preordered them both new)
Pokemon Soulsilver and Heartgold (complete), Avalon Code, Black Sigil, Radiant Historia, plenty of $40+ DS games.
Also many $5-10 games can become $30+ complete with case, book, all inserts including posters and registration cards, etc.
I'm less than 2 dozen away from completing my DS collection woot.
All RPGs and quality first party titles, not complete library (no Hanna Montanna, Spongebob, Madden, Call of Doodoo etc). Have pretty much every DS game by Square, Atlus, NamcoBandai, NIS, etc. Eg all the ones that matter.
Recently acquired Dragon Quest IV for $12 CIB yay.