before you can price it you'll need to get it authenticated and acquire a certificate of authenticity.
The fact that its on a napkin, don't expect it to fetch much....probably a hundred tops. A joe dimaggio autographed baseball with COA sitting on E-bay for about $250 with 30 minutes left as I type this.
http://cgi.ebay.com/JOE-DIMAGGIO-AUT...QQcmdZViewItem
and remember no matter what anybody tells you that it worth...its really only worth what someone is willing to pay for it.