I've heard through various grapevines that the lowest you can get on any station in the market is in the 500-1000 dollar an hour range. And it goes up, sometimes significantly, depending on what station, what time, etc...
As Steve said, most air at times when it's hard to sell spots. And since the stations that seem to run the most (talk) focus most of their energy on 6a-7p M-F it's almost a win win situation.
Sales can focus on selling their bread and butter (Whether it be Mark Davis, Gallagher, Bolton, etc) while still making money on what might otherwise be a 'gimme' daypart...
Not to mention the pressure it creates on the rest of the weekend inventory. IOW, Let's say you have 12 hours a day of 12 spots an hour (using those because I know the answer and don't have to do advanced math like "multiplication" to get an answer) Instead of selling 144 spots, if you do infomercials for even half that time, you know only need to sell 72 spots. Same demand, lower inventory, generally means higher rates...