I sold one in '87 for $1500.00, it had new valve guides in it and ran strong. It was Guards red and would only be worth 10 to 15,000.00 now due to non original paint and front fender. It was a late '74 model.
An original one like in the Barber museum would bring between 25,000 and 30,000.
They're really only worth what someone is willing to pay for it.
An ebay auction is a good way to find what it is worth this year.