I would look on ebay for similar comforters and see what they sold for. There is an option to search completed listings. I imagine ebay will be the best market for this type of thing.
As for value, it depends on what someone is willing to pay. With stains, the value will decrease. If it is a comforter with the high loft batting, it may not be from the 70s. The sheet sets I had as a child in the 70s had thermal blankets and coverlets, not comforters. The sheets my kids had in the 90s and more recently have the thick poly comforters.