A few problems there:
1. I believe the price pulled there is from the current exchange rate, which may be lower for the US release. It's gotten wide enough that we aren't getting a $250 Wii U with $300 Deluxe, but Deluxe would probably be $400 if they followed it fully.
2. You can't use more than 2 Wii U Game Pads per system, so it'd be... $175 for a "full set" at worst anyway, though really you'd need to pay $40-50 per Pro Controller, though the 360/PS3 controllers seem to reliably go for $30-ish anyway.
3. They will support Wii Remotes for multiplayer depending on the game, IE the Nintendo Land games, so for some games you already have a full set if you had one for Wii. Mario Party sounds like a prime target for this.
I also tend to find these
full price arguments disingenuous, though at least you're actively using the 'I want to play party games' angle here rather than just acting as if you MUST have a full set because.