Actually, the traditional ad-agency commission is 15%, but Google isn't an ad agency or media-buying service, so the comparison is irrelevant. It would make more sense to compare Google to a rep firm, but even that wouldn't be entirely accurate because Google is both selling and serving ads. What's more, Google is adding value by acting as an aggregator and advertising exchange.
In any case, it's pointless to worry about who's getting what percentage payout, because:
1) We know that Google is paying out 77% overall, but that's all we really know or will know.
2) Our contract doesn't specify a percentage.
The numbers that really matter are eCPM (which determines whether AdSense is worthwhile compared to other revenue streams) and total revenues (which pay the rent and mortgage). Our contract with Google gives us the freedom to pull AdSense code from our pages at any time, for any reason, so we don't need to worry about being locked into a relationship that's unproductive--if Google doesn't provide us with compensation that meets our expectations, we can say "You're fired" or simply remove AdSense ad units from our pages without saying anything, as we see fit.