Amazon is also generally quite a bit cheaper than other places.
Gamestop USA has it at $29.99
Wal-Mart USA has it at even more than that, $39.82
Toys R Us USA has it at $29.99
I could keep going, but I think that's enough of a cross section to show $29.99 is the standard price.
Not like it's a big deal though, there'll be a 2 for $40 coming up again sooner or later. But if you're that set on getting it at $19.99, there's always Goomba