Harcorehosting, OK heres my opinion. First, don't think your the first to have this idea.. It's a good thought, I and many others have talked about something like this. There are several reasons why this won't work. First, it is already a very very low percentage of surfers that actually ckick on a Free Host header or footer. Now to cut that back even further would be even worse. The small surcharge you mention, would barely bring you back to where you were before giving up the extra real estate. Because in order to do this per gb, you now would have to do all different types of tracking, billing, and so fourth... This extra work would just eat up the bandwidth surcharge you would collect. Also now your basically getting into Paid hosting? Well its 2:15 am here and I can hardly stay awake, I hope this shed just a dim light on the topic..
Remember this is just my middle of the night opinion.
------------------
Smile and Be Happy
Lightning Free Hosting