Centipede: Infestation 3DS - $9.04 @ Amazon ($8.87 Wii Version)

Great co-op arcade shooting fun. Decent price. My kmart had it in clearance for $9.99

Infestation really is a great game, though it is a fairly short one. I've played through it all on the Wii version, and while it is an enjoyable (albeit easy) ride, it only lasts about 5-8 hours tops. The co-op mode extends the game play significantly if that is an option. For the Wii, I would highly recommend using a classic controller for true twin stick shooter controls.

Also, I would recommend checking out your local Gamestop, used copies go for $4.99 now (Wii Version, $6.99 for the 3DS), and the ones I've seen are all in pretty good (near new) shape.

I think this is $9.98 at TRU as well, has been for like half a year.