I intended to ask about back issue pricing in general, but as always it's difficult to convey the proper meaning online.
No worries. Your best bet is old Overstreet Price Guides. It’ll give you some idea of the history of values, and the market reports will be helpful to capture the zeitgeist of the moment.
The caveats I’d give you are this: pre-2000, Overstreet was really slow to adjust pricing on books that blew up. It took them several years to catch up on pricing McFarlane Amazing Spider-Man’s, for example. The problem continues post-2000, now magnified by the fact that books blow up and cool down much faster in the internet age.
Second best options may be old Comic Buyer’s Guide issues and ads, and other old ads, like the Mile High ads that ran in comics in the 80’s and 90’s.