See, this is the marketing trick. It is employed by Hotels and Rental Car-Companies in conjunction with FF programmes.

So, double occupancy in double room for single room price is all well, but which single room price? Naturally the rack rates. And the rack rates are always beaten by what you get on the internet, via a TA or with corporate rates. Upgrades I get rather frequently on Radisson SAS hotels. My corp rate gives me a single room, but more often than not I end up in a twin room (how useful to have two separate beds...), sometimes to Business Class. Perks like late check-out also works (even when the hotel is full, and they are less than enthusiastic about it).

Rental cars are the same. 20%-30% reduction of price... on rack rates, which once more are a lot higher than what you get on the web. That said, on Hertz the EBG CDP gives me a net effect of roughly 10%-20% compared to not having a CDP, so they are not too cheap. Avis tends to be a different story.