This does look promising but it is in no way cheaper than existing options. I am currently using a family plan from AT&T to get 1GB (10GB shared among 10 to be precise) + unlimited calls+text for $27.17 per month.
Are you on a single plan with 9 other people? If so, that really isn't a fair comparison. A single line on T-Mobile is a fair comparison.
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.)