Hi Bitterroot
I can't give you an exact ammount, but I am sure it will degrade the value of the rifle. I don't know how many times I have found clean pre-64 Winchester #94's at small gun shops, only to find the left side of the reciever has been drilled with half a dozen holes for a side scope mount. My opinion, it really hurts the value, though in your case, the drilling on top of the reciever might not be as bad, and can be pretty well hidden with flush filler screws.
Larry