I recall coming across economy class tickets operated on BA where the cash due on BA award ticket redemptions was +/- 10% of the all-in price of the lowest regular paid economy class ticket for mileage earning flights on the same city pair on the same day -- this was a result of the "taxes and fees" on miles+cash BA bookings on BA flights.
A growing list of airlines have effectively turned a growing number of award redemptions for "free flights" into an opportunity to add revenue to their financial statements.