H Larry- I had a problem with runaway data use while I had tethering turned on here in Massachusetts last year. I think AT&T had a problem going on because I did get a refund. Meanwhile, I installed Dataman Pro which tracks data usage over time and by app. It runs in the background and has given me reliable results. I'm back to tethering in Massachusetts and the Dataman Pro tracking figure lines up with the reports back from AT&T. It can display data usage by the hour and the apps that used the data during that hour.

Oh... yeah.. 100mb doesn't go very far. This month I have a 5GB plan and have consumed 1.8GB so far having streamed some video news, some audio and doing lots of internet surfing. My wife too. Plus navigating to various places around the northeast using various road mapping apps.