A used boat is only worth what someone will pay for it. Few things to look at to get an idea.
#1 Boat
#2 Trailer
#3 Motor.
#4 Extras
Here's how I did the break down on mine I purchased.
The boat... by itself is well lets say this, mine's about $2500 for a 16yr old 20' Champion. That's just the boat.
The trailer....Is it single or tandem? Yes this makes a difference and even more so if it has trailer brakes. That figures into the price as well as the condition of the trailer. Mine was tandem in good shape with working trailer brakes....figure maybe $1800
The motor......THIS is where the money is. How many hrs? Newer 2 stroke, 4 stroke, or older 2 Stroke. Mine less then 500 hrs with maint. papers on a Merc OptiMax 225 with a SS 4 blade prop(about $250 in the condition it's in) from dealer of what has been serviced and when. It was well maintained and serviced. And the motor was one of the main things I had to have as our lakes require newer clean motors since older 2 strokes won't be permitted on certain lakes so this was a big deal. Figured mine is worth somewhere between $8-9000.
Extras, trolling motor....$500.
So with that said figured the way it sat at the dealer it was worth about $13,500 but it was listed quite a bit more, but paid a little bit less then what I figured.
An $8500 boat......do a break down and see what you can figure out. Extra's are extras, old electronics are just that old. What's that mean, well they loose a lot of value quick. Figure out what the boat by itself would be in the condition its in, the trailer as well, and then the motor. The cost of those 3 should give you a good idea about what you should pay. AND ALWAYS TEST DRIVE A BOAT ON A LAKE BEFORE BUYING. If they won't agree, well then either they don't really want to sell it that bad or they are hiding something.
Of course, the real price of a used boat is what someone is really willing to pay for it. Good luck on you search and future purchase.