DBDyess

"..... turning off the mifi/wifi when not in use, changing the email setting so it's not automatically feeding in, etc. We have a bundled account with the phone plus mifi, so when using the ipad or laptop at home, we use the mifi/wifi - both his phone and the wifi are using about the same. In less than 2 weeks we are already at 5.842! ...."

This part of your post has me wondering what you are using your mobile data for....

If you are using your cellular data plan to provide Internet connection for your computer at home, then I would expect your usage to be well over 100 GBS a month. 

Why are you turning OFF wifi?  This forces your phone to use cellular data?

15 GB use on your cell phone, if you don't have home wifi is NOT high.

Families that don't collectively go over 10 GBS a month, have an internet provider for their home with wifi and a large data cap. ( I have Cox cable and 300 GBS a month)