Somewhere over £1500, but it was a work bill.
I was using my work phone as a 3G modem, we were on a suitable Orange plan for that and our phones supported it with no hackery involved. Some of my colleagues did the same – if nothing else, it saved the company £10 or so a night on hotel wifi when we were away.
Company changes to Vodafone, we ask the right questions to managers about "do we still have unlimited data?" etc, met with a "just use your phone as before" response. So I did. Turns out we only got 500MB included, and I used a couple of GB that month as I was at a customer who won't allow us on their network. All excess charged at £1/MB.
Thankfully not my problem. I think the company managed to get half of it refunded via goodwill.