A couple of years ago some whiz invented a 'filter' that could compress a talk shows by removing the momentary silences. In an hour long show it could squeeze out an extra minute or so, which could then be sold by the station. Obviously radio stations liked this device. Rush was one of the only hosts with enough clout and chutzpah to complain. He insisted that his pauses were an important part of the show. I don't know if he was able to keep his show from getting the compression treatment.
Stations sells some ad time and hosts also sell some of their own time. It's a mix.
It's in use in our market; Rush calls it "Cashbox". He even mentioned it in his address before the NAB (was it the NAB he addressed a week or two back in Philly?) ...