I can't answer specifically how much it costs for a keybed. We would probably be shocked at how little it costs. Most members gripe about the Yamaha arranger keybed. I do notice the difference, but it doesn't prevent me from buying a Yamaha. Casio, Roland, Korg, all make decent keybeds and at competitive prices, so why doesn't Yamaha. They do have decent keybeds for their other non-arranger models.
Starkeeper