How is the best way to “market” a show prep service? How much should be charged? Can it be a “weekly” service? Are there any "syndication" services?
The first thing you have to determine is what makes your prep service unique. The more unique it is, the more you can charge. If it's simply another aggregate of information that is readily available everywhere, then charging anything will be tough. If you're providing truly unique, original content for shows, then you can charge big bucks for that.
Most prep services are simply an aggregate of the days' headlines and weird news. Spend one afternoon setting up an iGoogle page, or a well-organized Twitter account with lists and all, and you would have the equivalent of 90% of what prep services offer.
With that, if you have something unique to offer, throw it out as a freebie for 1-2 months (yes, months) for shows to try. If it proves to be consistent over the long term, people will buy it up. Radio is looking for original content.