I wouldn't pay the price of an unaltered, original gun, but certainly somewhere between the value of the gun in its original, unaltered grade and what an original unaltered gun would cost in the new configuration. Somehow I get the feeling that the seller is going to insist that the gun is in unaltered original condition and ask for a premium price in accordance.... Then it will depend on how much you want the gun...
I agree with what James says about the costs of upgrade versus the value of the gun afterwards.
Gil