The best source of fares from back then is probably old issues of the Official Airline Guide. Perhaps a nearby public library has some in its archives. If not, you could try contacting them (contact info on www.oag.com) to see if they can help, especially if there's a purpose behind your question besides random curiosity. Some travel agents may still have some lying around, too.

BTW, I recall paying about $600 for BOS-ORD round trips fairly regularly in the 1989-1991 time frame. That was economy class, plenty of advance notice, but no Saturday night stay - which was required for the lowest fares back in the day. (Fortunately, I could parlay that ticket into 5,500 DL miles and a seat in front. That was my introduction to the FF "game," but a topic for a different thread.)