Unless you need to use a PC from some distance away I really don't see why you would want a wireless keyboard. Most spend their entire lives sat on a desk where the wireless feature is utterly redundant. However, more importantly, why are Wireless keyboards SO expensive? A cheapy media keyboard with a wire is £10. A cheapy wireless one is £25. A fairly decent media keyboard is £20. For the wireless equivalent you are getting on for £50 - maybe more. Bluetooth adaptors retail for about £5 so why the enormous hike? In terms of materials I can't imagine there is more than £2-3 between them.
Is this just a case of the sort of people who feel the need to have a wireless keyboard sat permanently on their desk also being the sort of people perfectly willing to shell out £50 for the privilege of said fixed-wireless device so the manufacturers are simply making hay whilst the sun shines?