Or contact the airline. They need historic fare information for refund calculation reasons. It may be their refund or accounting department if more than a couple of months old.
ATPCo - Airline Tariff Publishing Company (should be googlable) will have historic price information for most network airlines. Not sure if they would be willing to give non-customers the information.
In both cases I suspect that you would only get published prices, rather than fare quotes so I hope the journey was a simple out and back!