Sounds like the MiFi is definitely the way to go. I didn't know they had such inexpensive plans for those. Not bad!

Are you certain it's Google Music that's taking up the most data? Have you checked the phone's logs?

I feel your pain, though. I just hit my cap for the first time this month (thankfully I hit the end of my billing cycle in a day or two). It's helpful having Android's data usage by app so I can see what's taking up the most data. This month, for example, it was the app I use for monitoring my surveillance cameras at home. Apparently it was receiving tremendous amounts of data.

Usually, my biggest data hog is my podcatcher. That one is easily fixed by downloading only over WiFi, but I usually don't have an issue of reaching my cap, so I don't worry about it. It was just this past month that was annoying.