Not true. The factors that affect what you pay is dictated not only by your QS but the QS of the advertiser below you and his bid. That statement would not even be true if you always had the same advertiser below you. But going from 5 to 10, there's little chance of that happening as that would seriously positively affect your ad rank. The only instance where you might save 50% is if you were already in top position. Download the Adwords FAQ. Page 11 has the cost formula. Play around with some test data. But keep in mind that in the real world, things are not static. Advertisers come and go, everybody's QS changes with each search, bids are changed, new ads are tried.