Depends. A online radio station can be started for as little as the cost of your internet connection, or as much as $50,000 and more.
Commercial radio is heavily regulated thus is expensive to enter into. The commercials are what a commercial radio station lives on. And as they get more listeners and more popular, the more they can sell there advertising space for.
Example the #1 radio station in Edmonton, Alberta, Canada sells 1 - 30 second commercial for $150.00. Yet the #5 station only gets $75.00 and the #10 station is lucky to get $35.00 a spot. And usually you have to sign for a minimum run of weeks and spots. The average small campaign is worth about $4000.00