That is hard to say since Kelly Blue Book doesn't list the Saleen. It probably wouldn't be worth a lot to the general public but, someone into Mustangs and Saleens in particular would probably pay the $18,000. It is gonna be a case of finding just the right buyer for the car. He might want to look at joining some Mustang forums and posting it in their classified sections.
Here are a few links he might find useful.
Saleen Chat
http://www.saleenchat.com/
Saleen Mustang Owners Group
http://clubs.hemmings.com/clubsites/smog/
Saleen Owners and Enthusiasts
http://www.saleenclub.net/saleenshow.php