My guess is 299$ - 399$. Nintendo knows that it can't be too high (599 US Dollars anyone?), but it also can't be too low because it will mean less power = no solid next gen console.
Anyway, if they stay at that price, the PS4 and Xbox 720 will have to be more expensive to beat that hardware, and it will be too expensive and they will fail.
Fanboys who says that it will be beaten by PS4 and Xbox 720 easily are wrong, it will be too expensive, so as long Nintendo makes it somewhere between 300-400 they will be just fine.
Anything over $300 would be ridiculous. Here's why:
SNES was originally $199, and the Genesis (its biggest rival) cost the same price. The SNES was more powerful than the Genesis.
N64 was $199, and the Playstation was $299 at launch. The N64 was more powerful than the Playstation.
Gamecube was $199, and the PS2 was $299 at launch. The Gamecube was more powerful than the PS2.
Now, if the rumors are true that the Wii U is anywhere from 50%-500% more powerful than a PS3, that doesn't necessarily mean that it will be the most expensive console. Nintendo has never had the most expensive console on the market. Sony learned from the PS3 that consumers won't pay $500 for technology that won't mature for another 3 years. Microsoft knows that the lowest they'll go on a console launch is $299. This means that Nintendo will NOT have a $300 launch price, because they strive for the lowest price (and when they don't-3DS, anyone?-they suffer
badly). This leaves my final guesstimate at anywhere from $250-$280.