It really depends on your camera and how you configure it.
If you use the largest image size, usually VGA, and a moderate compression setting you'll probably have an average image size in the 30-50KB range. I recently looked at a day's worth of images that we store and the average is about 50KB per image.
If you send us 1 image per second and they are 50KB, this will mean about 123GB per month:
(50*1024 * 60 * 60 * 24 * 30)/(1024 * 1024 * 1024) = 123
Note that this is upload bandwidth.
Another option is to send images much less often but augment it with your camera's built in motion detection. You can have the camera send an image per minute on a schedule but then blast out more images only when it detects motion. If you tune the motion detection on the camera, this will result in a lot less bandwidth usage.
Hope this helps!