That is called remnant advertising. You just say stick the ad in when ever you can and you get a discount rate.
OTOH, with the number of viewers he has, specifically being a sponsor has to command significant bucks. He’s worth serious money to FOX.
Not only that. On local TV, especially in the heartland of America and in the south, where Beck is extremely popular, smart businessmen, can sponsor spots on Glenn Beck's show, with tag lines like “Proud Sponsor of Glenn Beck”, and see their business go through the roof.
Plus even when ads are not specifically aired on Beck, the 3 million viewers that Beck brings to Fox, inevitably spill over onto subsequent programs, which then make even more money for Fox. There is simply no other program on cable news that even comes close tio Glenn Beck in ratings, and few programs on networks news that do.