According to the Foxtel on XBOX 360 FAQ this is the data usage rates using Foxtel on XBOX 360 over your internet connection
The approximate data usage used when viewing one hour of Live TV is:High Quality: 800MB/hrMedium Quality: 540MB/hrLow Quality: 360MB/hr
For Telstra Bigpond customers, the usage is unmetered or unlimited, however for other ISPs, your usage is charged.
For those looking to use Foxtel on XBOX, it does work great on ADSL(2+) and is on par with standard Foxtel service. You do get that same variety of channel although some changes and shows are unavailable on this service. For a list of the packages available, see here.