No one has any idea. Some may be free for all we know, they may be $5 for old ones, more for newer ones, or flat pricing across the board. We won't have a clue until either Nintendo releases the info or we have the consoles in our hands.
I can't see them exceding more than $5 for any game. The old N64 games are $15, which I have no idea why. But the NES and SNES games should be about $1-$3.