For the past year, I've been trying to keep close tabs on my bandwidth usage so that I can properly cost out what it would cost to switch over to a colocated server.
I put 99% of my images on a subdomain to improve loading times on the site as well as to seperate out the bandwidth for tracking purposes.
Right now, I serve between 800Mb and 1.2Gb a month of HTML, and between 2Gb and 6Gb of all non-HTML files per month.
Now that I've got my stats fairly locked down, I need to start doing some heavy-duty bandwidth estimations on some of the other items that I'm planning on offering. I also need to start pricing out 1U servers.