The hottest products in talk radio are coming from national sources. It works in most markets.
This sounds like a classic case of getting cause and effect mixed up. The hottest products in talk radio being national is not the cause of talk radio stations' success; it's a reflection of the escalating costs of doing talk radio at the local level, especially at the small market stations, and the aging demographic of the programming. If you look at Hannity, for example, he rides your ratings more than anything else. A successful talk station will continue to be successful if it puts Hannity on in afternoon drive. The struggling talkers, however, don't get much, if any, help from him. That can be said for many of the national products.
Most of those shows are expensive to carry. Most are not barter.
However, they're still cheaper, and significantly cheaper, than hiring a local talent. If you have Limbaugh and can follow him with Hannity or a local talent, you're almost sure to select Hannity because he's virtually guaranteed to hold the audience at less cost than a local host, which is more of a crap shoot with the audience.