To buy one outright... several hundred pounds.
With a phone contract... well it varies, but as an example, O2* will let you have one for a mere £140 provided you agree to give them £47 per month for the next two years. Seems a bit steep to me.
* other networks are available.
Personally, I get on just fine with my Nokia... something or other. There's no mobile signal at work anyway, and I have a computer at home for all the internet-y stuff.
