There is a ScreenSteps page at WPI on
Measuring Bandwidth Use which you might find useful.
The
FMS Whitepaper Rev.A also includes a table of expected camera use which I've reproduced below for convenience.
The FMS table suggests peak use of 4.3Mbs for the 640x480; 10fps; compression 30 configuration described below, somewhat more than the 3Mbs seen on the dashboard.
Unsurprisingly we found that the actual bandwidth used varies by about 20-30% depending on the specific image sequence being compressed. However counter-intuitively (at least to me) we noticed that bandwidth was higher for steady images and lower while the camera was moving quickly.
Edit: Daniel's explanation of MJPEG above supports this observation. In a moving image each frame is blurry so has less detail and can therefore be compressed more. But there is no compression gain from the frame-to-frame similarity of a steady image if each frame is compressed independently.