What R&D told me was that each log record is roughly 850 bytes to 1000 bytes when exported with Log Exporter, with the average being 950.
Since I don't have a way to test this, what I can give you is the stats from a family of four who making use of our Internet at home.
During the last 24 hours or so, my family of four generated about 30GB of traffic through my gateway, which generated roughly 80,000 logs...with most of the blades enabled and most (not all) things logged.
Using these estimates--and they are just that--I would be exporting 72.5mb a day in traffic via Log Exporter.
However, this is based on the traffic patterns of my family over the course of one day.
Lots of things will impact the real numbers, as I said.