The real question is what has Apple done to prevent people from using a $10 set of four plain black nylon casters instead of the $550 official casters with the brushed aluminum "Apple look"--that are still missing a very basic feature of a $20 set of furniture casters.
Even the fancy ball casters with faux-antique bronze finish and soft rubber treads are only about $12 each. A heavy-duty top-plate caster, with solid nylon wheel and dual ball bearings, and brake--all the bells, and all the whistles--might run $30 each.
It's so audaciously greedy that it's insulting to people who wouldn't even consider buying it.
You could even put your Mac in a top-of-the-line toddler stroller for $300, and that would be forward compatible with your next crazily-pampered desktop computer.