In the early 1980's, there were quite a few $99 East Coast to West Coast fares. PeoplExpress had a $99 SFO-BRU and EWR-BRU fare but that was promotional and lasted a few weeks.

During that same time period, IAH-LAX flights were often $250 round trip, occasionally $300.

Summer US-Europe flights were pricey in comparison, maybe $600-800 round trip in coach.

1970's? I think they were higher than the 1980's but I threw away all my tickets from that era. I still have a stack of tickets from the 1980's.