When I flew the Bus, I was told it was more about the weight penalty than cost (2000 lbs sound right?).
Normally NBD. But one Autumn afternoon, did an ORD-Indy-ORD turn. Brakes were pretty warm at the gate. For reasons I've forgotten, we had to taxi almost two miles to takeoff.
The brakes were just under the limit when we rolled. Capt kept the gear down for about 30 seconds after liftoff. Gear up, checked them, starting to cool off. Just as we level off, BONG! ECAM for hot brakes.
Slowed down and dropped the gear. We were about FL200. I figured in a 200+ kt slipstream that was below freezing, a minute or two would do it.
Nope. Had the gear down all the way to Chicago. I was amazed.