Everyone would have different uses for them so therefore, everyone would have different cost ceilings. I think once they are released and different industries start putting out different software packages, then this question would be easier to answer.
About ?300 max, maybe ?350 incl taxes. But it would wholly depend on whether it would be useful to have in my life, rather than be something I'd play around with for a few weeks and end up gathering dust after that.
I'll pay up to $1999 (USD), simply because I desperately want Microsoft to be a success with this thing. This can change the world of computing if it's successful. For the love of humanity, MS, *market* this! Like, Halo 3 levels of hype, please!
I'll pay whatever they ask. I'm usually not an early adopter but this is the most exciting thing I've seen in tech that I can remember. If MS can get it right, the possibilities are practically limitless and it will fundamentally change people's lives.