http://beta.beckett.com/
From what I've seen, cards from the boom (early 90s) have dropped dramatically from when I kept track, which was around that time. The over saturation to meet market demand killed most of those cards.
The mid 80's ones should treat you well.
Not the easiest method, but I'd check ebay as well, see what people have cards listed for, or where the auction is at... if there's no offers, you know the price isn't right.