Hi BJ, I too have a Roku with an Xfinity plan (not the Old peoples plan, yet) and I think you should not be exceeding the 1000gb(1Tb) cap. I believe streaming Xfinity data is free when using the X1 Beta Channel.
I also have an nVidia Shield that I do a lot of streaming on, as well as the Roku. Generally though I only use the Roku for X1 Streaming. The only time I use a lot of streaming data from Xfinity is when I use the nVidia.
You can check your data usage by logging onto your Xfinity account and select SERVICES at the top (near center). Scroll down to the INTERNET choice and select it. Scroll down from there and you should see a graph that shows the streaming data usage Xfinity has for you.
You might also want to inquire with Xfinity why you are getting these charges. Like I said, I think it should be free when using X1 Beta Stream Channel. On my system I have checked that out and it is true for me.
Thanks for the tip. I don't know if I qualify for the Old Peoples Plan (I'm 73) so I'm going to look into it.