So sorry for the long wait.
Regarding your question, the answer is: it depends on how you use your cloudBit.
If you’re just reading data and triggering stuff with IFTTT or through our API, we’d say you’ll be using ~13-20MB per day, since the cloudBit will be sending one input (amplitude) read every 200ms (the default).
Now, if you’re writing to your cloudBit, e.g. through the HTTP API or have explicitly changed the input read interval (ms), these values will change accordingly.
Say you set the input read interval to 2000ms and you’re not writing to the cloudBit. You’d be using ~1.3-2MB per day.
Attention: have in mind that littleBits cannot guarantee how your ISP measures and charges you.