A lot of big airlines have set costs whatever the number of brake units replaced. The only difference in cost is that of man hours to change the units. The SOP at my airline on the A321 is to aim for Flap 3 landings, idle reverse, with auto brake on LO until taxi speed. This is of course unless performance dictates otherwise. Someone clever who knows more about overheads than me has done the maths, and I'm fine with that. As alluded to by someone earlier, I just wish more pilots would leave the auto brake in until it needs to be disconnected. This helps even out the brake temperatures for the turnaround, and also evens the brake/tyre wear.