Let's see..
3DS = $249 (an educated guess as to the cost of making a 3DS was said to be like $103)
PS Vita wi-fi model = $249 / 3G model = $299
Despite being obvious, Sony did state that it would sell PS Vita at a loss. However, it expects to begin to make up a profit off of it in 3 years.
I think $350 would be the best price point since it would still be considered affordable and competitively priced for a next-gen home console. In addition, that would still give Nintendo some room to slash the price a bit at least 2 years down to be in a position to take on Sony and Microsoft's next machine's head on.
However, in order to make a profit off of the hardware from the get-go, I'd say $399 (even though it doesn't sound very Nintendo-like) is what Nintendo's may to have to go with.