Google actually provided exactly this service (for all Google-based ads, which is a high %) for a little over a year before it was discontinued, supposedly because nobody actually used it.
It was low enough that it wasn't really a factor when determining what to price your ad-free pages at (most people went with 1-2 cents per load) -- most people just priced at whatever average your ads were pulling in currently without worrying about the cut, since it was a fraction of a cent.