"If you purchase a data plan from a carrier with, say, a 2-Gigabyte data plan, how much can you use your smart phone or tablet before consuming all your data?" asks Jim Hamm. He goes on with some useful info. " This link gives some estimates, which you may find helpful.
"If you like to listen to music while you're checking the internet (while using your cellular plan), this article gives you a good idea of how much data streaming music consumes."
Keep reading; Jim sends us that good word FREE. "If I'm using a cellular connection I usually don't listen to streaming music so I don't consume all my data. But if I did, I'd listen to Pandora (which I do when I'm connected to wifi) and I find it uses a maximum of 64kbps, which is fairly low quality, but adequate (at least for my ears). Of course, Pandora provides a free service, so I'm not complaining."