I was recently talking with my father about his time in the marines during Vietnam. He was stationed in Japan in 1972 and attended the Ali v. Foster fight in April 1st in Tokyo. We ended up digging through some of his old belongings and found his ticket from that day. I don't know if there are any memorabilia enthusiasts or experts on this board that could possibly give me any information about the rarity or value of a ticket from that fight.
Looking at completed listings on ebay, I'd be surprised if you were able to get $50 for it. You can get a very nice Ali-Liston or Ali-Frazier ticket for just a few hundred bucks and there isn't nearly as much demand for an Ali-Foster (rare as it may be).