Too much, especially if the scope is from the same era. While good enough guns the 80's and 90's made guns do not bring a lot of money. You can buy a standard grade pre-64 for $600-$700. The Classics made 1994-2006 with the pre-64 style extractors might bring about the same.
Some people ask more, but the push feed rifles made during the 80's and 90's generally sell for $300-$400. A Leupold, if made during the same time adds no more than $100 to the value. If it is a newer production Leupold then maybe $200, it just depends on exactly which Leupold.
If it is a Varix-I or Varix-II, it would add nothing to me. Good scopes in their day, but newer scopes are much improved. Leupold upgraded and changed the name to VX-I or VX-II sometime in the 80's. Those are a little better, especially the VX-II. The most recent scopes dropped the roman numerals and are now VX-1 or VX-2. Basically if the scope uses roman numerals, there are much better scopes today and it shouldn't add much if anything to the value.