Generally speaking, champagne does not age well (see https://www.winespectator.com/articles/does-champagne-age-well for example) so while I am no specialist on vintage spirits I would not depend on the contents of the bottle to have much if any value.
You can look at completed auctions on ebay to get one estimate of value
https://www.ebay.com/sch/i.html?_nkw=PERRIER+JOUET+VINTAGE+1985+CHAMPAGNE&_sacat=0&_from=R40&rt=nc&LH_Complete=1