If L-L.com vs LLL.com is any indication, N-N.com should be 1/10 to 1/20 of NN.com
But probably it is further reduced by the fact that with N-N it looks like "n minus n" rather than "n dash n".
Fair range could be anywhere from $10,000 to $100,000. Yes, 7-6.com sold for really cheap at $2k.