Why are people assuming that the Wii U might be "underpowered" if it costs $300? Since when do we judge a system's power based on it's price?
$200 - $300 was the standard launch price of consoles for many years. The PS1, PS2, and Xbox were all launched at $300, and they were vastly different in terms of power. The PS2 was more powerful than the PS1, yet it launched at the same $300 price. The Xbox was more powerful than the PS2, yet it also launched at $300.
As the cost of components goes down, then the cost of the console goes down. We see this all the time. Manufacturers drop the prices of their older consoles so they can launch the next one at a comparable price. It's how the industry has always done things.
The only reason the PS3 cost $500 is because the cost of components was so expensive at the time (Blu-Ray, Cell processor, etc.). Nintendo doesn't have to go the route Sony did. They can still make a powerful console without going all-out.
We can't have these preconceived notions of how powerful the Wii U will be just because it might cost $300.
If anything, the Xbox 360 and Wii are way overpriced for how old they are. The Wii was technically outdated and overpriced the day it came out. Xbox 360 is showing it's age as well, the components must be dirt cheap by now (by PC standards). PS3, on the other hand, can possibly last a few more years, simply because of how advanced that Cell processor is.
There is no reason for the Xbox 360 to still cost $300. It should be under $200 by now. And the Wii should be about $99.