£500 or so. If you were selling the whole package, then possibly a grand with the engine and console, etc.
Check out Ebay, Boats and Outboards, FAFB to gauge prices. Once the season picks up next month prices will increase.
As a guide - here's an Avon S400 (SIB) on a Snipe road trailer 1985 in what looks to be very good condition. Starting at £300. He's also selling a mint 25hp Mariner 2-stroke of the same vintage separately and it's already made £550 with 5 days to go.
Avon S400 Inflatable Sports Boat and Snipe Road Trailer | eBay
25HP Yamaha/Mariner 2-stroke Outboard | eBay