I haven't recorded internet usage, I was once on a limited usage contract but I used it pretty quick (not on cRPG though and I can't remember what the max allowance was).
Your best bet would be to download some software to monitor and record you internet usage. Then you can work out how much you are using during cRPG.
From a quick Google I found
http://www.hootech.com/NetMeter/Looks like it will do what you need (but obviously I haven't tried it). I'm sure a quick search will find you plenty of other programs dong the same job.