keithmt, I'd suggest countering with a reply letter noting that - "you appreciate the offer, however with the noted interest you've already received so far, coupled with your original considered development plans, that to sell it at this point, you would be looking for something in the upper $XXXX range to part with it now."
No price has been mentioned from your side, let him counter his own offer, and then you can get a feel of where this could eventually lead to price/sale wise! Seems to keep your options more open this way.
Also as there are numerous tm's on the term, you might want to consider - 'who will come to pay more for it!!'

or, 'what headaches lie ahead!'
GL.