That is not completely true. There is one process using up some (very small) amount of data that you do not entirely control: the regular check for software updates.

But $20 per month? I have used a pre-paid SIM all my life and have never, ever gone over £5 in any single month. I usually top up by £10 once in every three months. That includes driving around with modRana downloading map tiles as I go along and a one-hour-long conference call once a week.