I take it you mean the value of the gun and not the plug (det pin)? its always difficult to put a price on any modelgun for so many reasons, condition of model etc but most important how much someone wants one.
Some yrs ago they used to sell retail for at least £700 (and that may still happen at times). £400 would be a ball park figure to value but as i say market depending, if you have am A1 condition model and boxed it would demand more.