Paul, not sure if you misread my message but average data usage is 2 GB/mo when Forwarding is turned on (not 20; there’s a decimal point in there).
I don’t personally have experience with GPRS, but there are resources out there that can give you some idea of what to expect for different Pi/GPRS setups. I think one or two of our forum users also has experience. However, each ISP differs slightly so you’ll want to contact your ISP to get the details of how to connect remotely in your case. Because of this I don’t want to link you to something that’ll end up being wrong in the case of your particular ISP.
I think you’re right that direct connections from Seedlink or Earthworm would be the way to go, since I think data transmission in those cases is binary, and per-month usage (if seedlink were connected 24/7) would drop to about the storage size on disk, ~450 MB per channel per month. If you go this route, data forwarding to the Shake servers can be turned off on the web front end, and the data can still be accessed at Seedlink port 18000 or wave server port 16032, or in SWARM at either of these ports.
If you have any further questions, let us know.