Check out NADAguides.com
I punched in the specs which show a generic 90 hp engine and with the trailer it says for a retail price of low: $4400 to high: $5500
http://www.nadaguides.com/Boats/1999/Aquasport-Inc/175-OSPREY-CC_/10010597/valuesKeep in mind that these are retail prices and I have found the NADAguide numbers are usually a fair amount higher than what can be had during a private, non retail sale.
How do you know the engine has 100 hours? That's pretty low.
That said, I think he is asking too much even though it looks like a nice boat.
Of course if you have been looking for one of these for a while and it is pristine, perhaps it is worth 6K to you or more..
I would run the numbers yourself, print out the sheet and show it to him and see what he says.
Dave