From what I have read the general thought is about 100mb per hour. As I far as I know Apple don't release any figure for the usage.
I bow to your knowledge. My understating was it streams around 360p which is where I calculated my figure and as you say it poorly forms on the video quality.
It really does depend on the quality of the feed Dave. You have quoted the lower end and I have quoted the higher end as I typically do when I know the question pertains to data usage. I would not have jumped in had I seen your post. Sorry about that.