Assuming it's a long-shaft? They retail new for about £4,600. Reckon it's got to be worth £2,800-£3,000 if it's mint.
You can go by the 20% off in year one and 10% there-after, but I think that's unrealistic, given there's still a decent market for mid-range 2-stroke outboards, especially with history, paperwork, etc. That's my tuppence-worth!