A surveyor will base his/her decision on market value upon searches for similar boat/similar location/similar condition actual selling price. It's not always possible to find a statistically valid number of boats of similar condition, age, location, etc so the surveyor will often provide a best estimate of market value based on these critierion. Surveyors also are privy to actual selling prices of boats which can be significantly different than the selling prices you see listed on various lists and URLs.
If you could look "across the board", you may be surprised to see how much lower actual selling prices are compared with asking prices. Hence, either or both of you may be working under a false pretense of value and you would be well-served to talk to the surveyor to get a better feel how he/she arrived at that figure.