Nice Idea, I run a "real" keyword value site. I check some numbers and we are very close. Good to see you didn't buy a list
My only concern would be Adsense TOS
"Code Modification
Any AdSense ad code or search box code must be pasted directly into Web pages without modification. AdSense participants are not allowed to alter any portion of the ad code or change the layout, behavior, targeting, or delivery of ads for any reason."
Have you check with adsense yet. If not why not do the whole code instead of ID only. I know some people would try to take advantage. But if you put a max size. It would only hurt them to give you the wrong size.
I think you need more assurance as to it's adsense acceptance.
Good Idea though.