Video data is only sent from the computer when an iPhone is actually connected and viewing the webcam.
When an iCamSource is idle it is sending a small amount of data (about 100 bytes) every 5 seconds or so to the iCam Broker Server to let it know it is still there. That equates to about 61 MB of data uploaded per month if it is just sitting there 24 hours a day, 7 days a week. When iCam or iCamWeb is connected, then naturally the image data being uploaded will be added to the total. Push notifications can also add to slightly to the total bandwidth.
If your bandwidth is limited or metered, I would suggest that you don't leave iCam or iCamWeb open viewing your webcam(s) for extended periods of time.