The value is, simply enough, what a willing buyer is willing to offer. There is no wide, global pricing mechanism for whiskeys, most of which are not rare -- they were industrially produced. So, even old ones generally are available though, obviously, not in their original numbers.
An Ebay listing may be as good a way as any of determining value. Or check out a website community such as www.straightbourbon.com, where enthusiasts gather and may be more interested in acquiring older bottlings than the general public.