In good condition an all original Zenit should be worth at least £120. Given that the pistol is not ideal for general use (unless you like wearing a motor cycle helmet when you are shooting), it is really only a collector's gun and collectors are fussy about condition. So I would value your gun with holes in the end cap at about £80 and the other one at £40-50.
(Incidentally, why are there holes in the end cap? Does it make it whistle when it flies towards you so that you have time to duck?)