How much data do you use on your iPhone now? How much will that change tethering? For me, I'll be using my iPad more like at lunch or something at work, than I would use my iPhone, but not a significant amount more. I won't stream audio for hours on the iPad when I wouldn't on the iPhone. Look at your data usage now and start there.
There are calculators on Sprints and Verizons sites that says if you stream music 1 hour a day for the entire month, that would be about 1.8 GB of data.