Massively is running a story about bandwidth costs for MMOs
and other virtual worlds. It's based on a post at the BBC
on the same subject which references a traffic analysis
(PDF) done for World of Warcraft
"If you're an average user on capped access, the odds are you have roughly 20Gbytes per month to allocate among all of your Internet usage (it varies depending on just where you are). For you, sucking back (for example) a 2GB World of Warcraft patch isn't something you can just do. It's something you have to plan for — and quite often you have to plan for in the following month. Even a 500MB download has to be handled with caution. MMOGs as a rule don't use a whole lot of bandwidth in actual operation. However, the quantity definitely rises in busy areas with lots of players, where there are large numbers of mobs, or on raids, and takes quite a much larger jump if you're using voice as well."