How long's a piece of string? It is only worth what you feel comfortable paying.It depends on the condition,whether or not it has radio gear in,and if it floats!Depends where you are,down this end of the world you would not pay as much as you would say in the London area.
They are wonderful models in the water,and will take quite rough conditions. If it is in average condition then around £175-£250
would be fair.If it is an excellent model with all the gear and ready to run, then expect to pay more.
Best of luck!
Mick F
