I think the US price is $50 up front, which gets you the communicator headset, bootup CD (or something equivalent), and 12 months of service. They still haven't announced what the monthly fee will be after that. But, for $50 (or whatever it will be in the UK), it's not that big of an investment to see what it'll be like. (it gives MS a year to iron out all the bugs in the system) The only thing that could burn gamers is the games themselves. If I pay £50 for an online game, and the network is crap, then the game is pretty useless.
That being said, I'm still looking forward to the experiment.