I think the OP was asking for a low cost keyboard with features similar to the SX900. It's really up to Yamaha's engineers to figure out how to hit that price point.
Eileen's comment about buttons for players with vision problems is well-taken. I don't presume to know how to make a keyboard easier to use for people with limited or no vision. Shucks, the UIs that I designed were horrible for the sighted.
I'm sure that the engineers take volume discounts, wiring, assembly, testing, etc. into account and cost is never as simple as just the component cost alone. Further, a very large portion of keybaord costs are development, including voice, multipad and style development.
All the best -- pj