It really depends on the stuff Nintendo puts into the console. We had rumors the console would equal the HD twins in power and other sources who said it will be a lot more powerful than those.
A console with graphical parity to PS3 / 360 could be sold at $299 while maintaining a profit (of course the tablet will add around $50 to the retail price) but if Nintendo decides to take a more sophisticated approach (Quad core CPU and 1.5+GB of Ram) I would expect a retail price of $349. I'd rather have them take a more conservative approach and focus on market expansion. Going for 3rd party support is nice but it could end in a console that can do anything but does nothing well.