Most radio stations make their money by selling advertising. How much an ad sells for is negotiated between the station and the advertiser. It is based mainly on how many ads the advertiser wants to buy and how many people are expected to here the ads when they air. Radio stations using frequencies between 87 and 92 MHz in the U.S. are reserved for educational, community and noncommercial use. They are considered non commercial stations and cannot sell advertising. Just like the PBS stations on TV. They can accept donations, and underwriting support. Also some Religious themed radio stations try to stay away from selling ads and rely on donations. Non-profit corporations who own radio stations also tend not to sell advertising.