You could start by looking at the average asking price of several 1801s and then just take 85 to 90% of that.
The 17 hull is 94% as long as the 18. The other percentage difference accounts for the smaller engine and was just a guestimate. You could get more specific for the engine (like 85 is 65% of 130...which is probably the average HP on an 1801) and come up with a formula that takes into consideration how the value of each individual component relates to the total package, and use that against existing 1801s as well.
But ultimately, the boat is only worth what a buyer is willing to pay.