Don't know what you mean by "fixed value", but BA's program is arguably the best value for USA domestic economy class flights if you live in a market that is well serviced by AA (or US) or AS.
Likewise, there is no need to ever fly AC, NH or SQ to use their programs to redeem for travel on any Star Alliance carrier, including UA.
I don't redeem for domestic economy class flights. Waste of Avios (or points).
Let's take BOS-LAX on AA. Would cost about $125 one way or 12,500 Avios. Value: 1 Avios = 1 cent. HORRIBLE!
Instead, I redeemed 4500 Avios for LIM-CUZ which goes for $180: 1 Avios = 4 cents. GREAT!!