If the NX costs more than $250, Nintendo might as well just sell their hardware division right now. A PS4 console bundled with a game is $349 this year, and that will probably drop to $299 next year (coinciding with a new marketing push for a slim model). I think the XBox One will also have a $299 slim model next year.
Even if the NX is $249 at launch next year, your average gamer, going to a BestBuy, will be deciding between a $249 NX with its library of 10 - 20 launch games, or a $299 PS4 / XBox One with a library of 100+ games, what choice do you think they are going to make?