Obviously depends on condition but I agree with others, somewhere in/around $20k. But you never know what is in his mind until you get him to name a price. See my thread from unsolicited offer to buy my '69 yesterday. Also, look at Hemmings and Excellence price values. Likewise, never hurts to look at completed auctions on eBay.
http://forums.pelicanparts.com/porsche-911-technical-forum/880863-longhood-value-unsolicited-offer-buy.html
Good Luck!