You have to have a smart phone that has a good data connection. Pandora (and the other streaming music apps) burn through data pretty quickly. It may be cheaper than SiriusXM, and the sound quality is considerably better (at least when streamed through a normal headphone connection to a stereo from my experience) , but you gotta watch that data usage.
You can stream pandora for free but you gotta put up with a lot of annoying commercials and they limit the number of times you can skip songs.