Well, a portable hard drive is $100. So if you don't have that and your Wii U doesn't have sufficient storage space for the game and you want Wii Fit U, then $50 is a good deal.
But if you have all that and a Balance Board and just want to buy a Fit Meter, then hey, free game! I look at it as more of a loyalty promotion than the "normal" price for the game.