I suppose it depends on which PPC provider you use as to what the best solution might be. I saw that PPC Pro recently added features for Google Adwords, and although early days it looks quite useful for those advertisers wanting some sort of self service.
My question would be what you get for the 15%, and the 35% is way too arbritrary. We've saved plenty of clients money on what they were spending but their expenditure is now more than before, but so is ROI and hassle is way less, what price do you place on that?
A cunning play on words!