With the August update, one can check the hourly internet usage for past 12 hours (I think) on XB1.
When playing FM5 online + chat, I am seeing about 100~150MB/hr of internet usage. I have no idea how much FM1~FM4 usage was but I suspect (with no evidence to back it up) that it was less.
Just on standby mode, XB1 uses 50~150kB/hr.
To check internet usage, go to Settings > Network > Internet Usage (at lower right corner of the selectable commands list).
If you have an internet cap, you may want to watch track your usage. It also conveniently shows the total usage for the month.