I think that can only be determined by the seller.
I notice a couple that went through E bay last ,were around the £400 mark .
I say that because the price of the last one was Buy it now £475 ,or best offer, and it sold under the "best offer " ,so I think it almost certainly went for £400. So it seems that`s about the top 2nd hand value to hope for ,as E bay usually gives the best indication on second hand detector values,unless of course you can get more.