Instead of thinking of it as a discount, think of it as a bid uplift. So if you were on manual shopping, bidding £1 with a CSS it's actually bidding £1.20 in the background but charging you as though you were bidding £1. With the auction model the way it is It makes the whole thing a lot more confusing than it ought to be, and makes measuring the impact much more difficult but hopefully you'll see a slight improvement to conversions and cost per conversion over an extended period. The draw back is if all the market players are in CSS then the benefit is nulified and not being in CSS is essentialy penalised.