If your streaming bitrate is 64 kbps, then you are using about 30 MB per hour, or 0.03 GB per hour, assuming some overhead.
If you've truly used 0.1 GB in half hour of just audio streaming and no other internet traffic, this would indicate a streaming rate of about 400-500 kbps, which doesn't make sense because Pandora certainly doesn't stream at such high bitrate. So, either you streamed for a lot longer than half hour or you were doing other activity on your phone at the same time that consumed data (navigation/maps, downloading apps, browsing websites, watching video, etc.).
Generally, audio streaming doesn't use a ton of data, unless you listen many many hours every day. With that said, a 1GB data plan is not suitable for frequent media streaming, IMO. 1GB of data equates to about 33 hours of streaming at 64 kbps.