Around 150MB per month. I've got WiFi almost everywhere I go and I use low-data apps (no streaming for me).
Around 600mb. Here in the UK I have to pay £10 for "The Works" which is a 1Gb of Mobile Usage/Tethering.
On this last day of my billing period, I have now used 4790MB.
I downloaded the Complete Led Zeppelin on iTunes Match, deleted it and downloaded it again in two days plus several other albums. Just wanted to see what it would take to use that much data in two days.
Now that I have iTunes Match, I expect to be using allot more than just 500MB per month like previously.
Do you not have wifi? You must have some home internet connection to match your songs from iTunes on your desktop/laptop right?