I just monitored a little bit with wireshark, it's about 1kB per request to the server.
So if you mine at 4GH/s you have about 1kB to get work and 1kB to report work each second.
If you mine at really lower speeds you probably get a little more data per hashing power because you don't try the whole block.
At 800 MH/s it is about 5 seconds per block so it probably won't matter, but my videocard is as slow as 60 MH/s and would take a little over a minute to complete a block and thus would generate a lot of stales.

Conclusion, at 800 MH/s I estimate it at 2 kB / 5 seconds, 86400 seconds a day, 17280 times 2 kB, 33.75 MB/day.