One of them is for 3DS and the other is for Wii.
Yeah, I realized that later. It would help if the OP specified that.
Man, I never thought Mass Attack would drop this low. I picked it up for $15 way back thinking it wouldn't drop any more than that (as most popular Nintendo franchises don't).
I think that myth has dropped some time ago. There are many Nintendo games that hold their value, and there are those that don't.