My Data Manages is a free app that enables you to know how much data you are spending.
Ranked in these QuestionsQuestion Ranking
Pro Very intuitive interface
This app's interface is very simple to understand and use.
Con Recent update shows inaccurate data usage
A recent update (early 2017) has caused the data usage to become unreliable. It often wildly overestimates data usage, but will occasionally underestimate as well.
Con Intrusive permissions
Requests permissions for precise GPS location, as well as reading the contacts, reading SMS, and reading call log (which are required for call and SMS tracking).