There are a lot of variables. Stations already pay copyright fees to ASCAP, BMI, and SESCAC for the rights to play the songs on radio.. Those copyright-royalty fees increase if the station streams too, and will go higher if the station doesn't make the best effort to 'geo-fence' their stream to only inside their market. (which costs even more money toward the streaming provider)
Then you add in the additional copyright-royalty cost directly related to on-line streaming; a Sound Exchange membership, which averages roughly that .18 per 100 songs played:
Radio Streaming Rates Likely To Remain In Limbo Until Next Year.
And all this is on top of the base cost of your programming via a streaming provider, which may charge a flat rate fee, or by number of consecutive active listeners per minute/hour/24 hour period. When it comes to paying streaming providers; the more popular your stream is; the higher this particular expense. (monthly flat rate plus a rate for each listener/stream)
Unless the station/group sales department sells ads specifically on the stream, the cost of streaming can be a back-breaker for smaller stations. Most smaller radio groups can barely keep their sales department going just selling traditional OTA radio, let alone have a separate individual or sales group selling spots on just streaming. The other downside; is your streaming sales potentially cannibalizes your radio sales, as they're either selling against each other, or throwing streaming spots in as a 'loss leader'-bonus to prized clients.
Oh, and don't forget one needs an automation system capable of covering the local radio spots with streamed spots or filler. That alone is an additional expense typically spread out over five years, plus it takes someone to schedule filler or sold spots on the stream.