If each of your users use more than 717MB of data and less than 1 GB, then AT&T is probably cheaper. How much would it cost you if one month you used a total of

If each of your users use more than 717MB of data and less than 1 GB, then AT&T is probably cheaper. How much would it cost you if one month you used a total of 20GB of data? If the data usage varies from month to month, Google would probably be less expensive. (I'm not sure if you are including taxes which would probably change things a bit, but you get the idea.)