I do not believe the actual cost and paid prices of "miles" are known in any public way.

When you pay QR for a flight and credit to BA's program QR credits your account from a bank of bulk purchased (This is also the way airline associated credit card companies do it) miles and BA puts those miles into your account hoping you redeem them poorly or they expire.

When you use BA Avios to book a QR flight BA pays them in a prearranged way & amount for the flight.