That's a subjective question, but here are some thoughts.
1) A beater to fair condition 91 or 92 NSX can be had for as little as $28K-$30K here in Texas. I bought a nice 91 for $30K two years ago. Don't know what they are worth in other markets.
2) Black/Ivory is a very rare color and, in my opinion, the prettiest (I owned one a few years ago).
3) The question you need to ask yourself is 'what premium am I willing to pay for low miles and a rare color?' In the end a 'fair' price is one that makes you happy with what you get.
4) For more context, you might pick up a Hemmings Motor News and check out the prices for similar year/milage cars. It is national and has a LOT of listings. It will also give you some leverage in negotiating with the seller if you decide the price is out of line, as you will be able to show him real evidence that other cars are selling for less. The Blue Book is also a referrence, but is always a little out of date for specialty cars.
5) If the car is truely perfect, you might try to negotiate a price closer to the average for that year. If he is wealthy, he will be trying to get a 'fair value' for the car as opposed to a specific amount he needs and may be persuaded by evidence that a fair market price is $5K or so lower than what he is asking. He may not really know what it is worth, as there are usually few for sale in all but the largest markets.
Good luck.