Whenever I can buy the game for $10 in the used games bin.
You do know that will take forever, right? Take a look at Mario Kart DS, that game has been going on since the beginning of the DS' arrival, and I still see it being sold for it's original price at GameStops. If it's Mario Kart, Metroid Hunters, Phantom Hourglass, Star Fox Command, or Pokemon (you know, all of Nintendo's key franchises) then expect to pay top dollar. And add Spirit Tracks to the list.
This game isn't "Barbie's Pony Ride", it's a top notch Nintendo exclusive. The only way you'll find it at $10 is when the successor to the DS comes out (and it isn't backwards compatible).