It varies depending on the streaming provider they choose, the content the station airs (stations got to pay extra if they want to stream major syndicated programming online), as well as the licensing deals they have with music providers. The cost of the streaming host varies depending on what options you want from them (bitrate options, max listeners, etc). For major station groups it's slightly cheaper per station if they negotiate one flat rate contract that allows them to stream all of their stations, but for a smaller group it can get pricey.
For more examples of what it costs, check the sites of streaming providers like StreamAudio.com, LiquidCompass.net or StreamtheWorld.com. But keep in mind that's just for the streaming host, there's more costs involved like license fees to the labels, as well as to syndicators like Premiere or Westwood if you want to carry their programming on the stream too.