Some of the value depends on where you are.
The model itself was excellent, maybe one of the best 25's built all time in terms of durability/reliability - all round a great motor.
Now if you are trying to sell it around Lake Meade where 2 strokes are banned because they are pollution spewing contraptions the local value is near zero.
If you are in an area where boats that could use that motor are restricted to less than 20 or less than 10 horses or whatever, again, local value is near zero.
If it's been run in salt water for most of it's life it might only bring half what a freshwater motor would.
Also, the big one will be the condition it's in - running like the day it left the factory, needs some work, not working etc will also greatly effect value.
I live in an area where a 25 2 stroke is a very popular motor with no restrictions on use. A local shop has one that has been "tech checked" (and the dealer offers a 90 day warranty on used motors). It's a 1986 electric long shaft, so in the same model run as yours and they have it listed for (about $1150 USD - $1450 Canuck Bucks).
So if your offer is in USD, I personally would consider that a "fair offer" in my area for a private sale with no warranty. As the buyer I would "expect" to have to service the waterpump, probably rebuild the carb, change the sparkplugs and maybe a couple other smaller items ($150 to $400 "more bucks" depending on whether I could do the work myself or would need to use a shop) - all costs that I wouldn't have buying a serviced engine which I would factor into the value.
So it depends where you are, the condition of the motor etc. Some places may place a higher value than I would. Others may offer you the scrap value based on the weight of the aluminum....