The RISC OS market is tiny, and can't possibly begin to approach the price of commodity PCs. A bespoke computer design is going to cost more than the mass produced PC world.
If you want a cheap computer, you can always get a PC. If you want a good computer then be prepared to pay a little more. We all wish Prosche would drop their prices to match that of a Fiesta, but it ain't going ot happen.