Bear with me, I'm going to put up some straw-man numbers. I don't have any knowledge of the radio advertising business. I've never listened to Rush, never wanted to, life's too short. Let's have fun and run some numbers.
The man makes $50,000,000 per year. Let's assume that's all for the radio show (is there a TV show? if so let's ignore it). He's broadcast on approximately 600 radio stations, 3 hours per day, 5 days per week. Let's assume he's only on about 220 days per year. Each hour of radio time gives about 9 minutes of ads (18 ads per hour).
That's:
$227,273 per day
$75,757 per hour
$8,418 per minute of ad time (9 minutes of ads per hour)
$4,209 per 30 second ad.
Spread this out over 600 stations and you get:
$7 per 30 second ad per station goes to pay Rush.
This is why that stations are saying his show is basically free. Each station would only have to generate $126 of ad revenue per hour to cover his show.
In the Baltimore radio market, 30 second ads are around $200, DC is almost $400. These numbers are from a quick 2 minute web search, I'm sure someone in the industry can provide a more accurate estimate. Let's say that each station charges $200 for a 30 second ad.
$200 - $7 = $193 per 30 second ad for the station
$3,474 per hour of ads for the station.
What are the station's fixed costs?
$500/hour if the station has 5 full time employees costing $100 per hour (salary + benefits)
What about power and equipment costs? Licensing?
Let's say about $2000 per hour cleared after fixed costs. Drop that by 90% and you have $200 remaining for profit instead of $2000.
If you cut his advertisers by 90%, the station and/or Clearchannel eats the cost. If you cut the number of stations running his show by 90%, Clearchannel eats the cost since he still requires $75,757 per hour to feed his ego. Unfortunately, having his show dropped by even 5 or 10 stations won't make much of a difference. Wingnut welfare will kick in and cover that difference.
It's all about dropping Clearchannel's advertisers and reducing the number of stations that broadcast his show.