I personally scanned a PS2 copy of SMC+ at Target, and it was $19.99. You need a nice/dumb cashier to get it for anything less (unless you've got a raincheck or something).
As others have pointed out, the GC version will only be $14.77 because it is, in fact, a Player's Choice. The game just came out for PS2 and XB so it's not a top seller yet.