My ISP provides a little App that sits in the taskbar and tells me how much of my allowance I've used. Some months I get as high as 60%
I told BT they can keep 1571 (I have an old answering machine that does the job for free). I signed up for their "pay for a year ahead" which saved a fair amount. Caller Ident was meant to be free with that package, but they charged me anyway until I spotted it and challenged them. The nice Indian lady arranged a refund in the following month and set it to "free" again.
Out here in the agricultural wilds there's no mobile phone signal to speak of, but BT is installing FTTC at the moment so I'm expecting to pay even more and switch to that.