It depends on how clean it is. Tandem trailer? Cover? New interior or old beat up? If it is real clean with a tandem trailer, low(ish) hours, cover, multiple props, etc..........I would say in the $10,000 range +/- $1,000. Take a look at what the late 80's Stokers with OMC's are selling for. This should give you a good idea!
Good Luck!