Not really uncommon considering a pair of discs are £400 new the last time I ordered them. Calipers are £400 each.
He's got a complete setup of calipers and disks and pads to sell that can bolt straight on at the moment, and you think it would make sense for him to sell 1 disk on its own and then leave him with 5 of the needed 6 bits to sell it as a bolt on kit?