Pretty simple really. Take the data downloaded and multiply by your contract fee. Pages are typically anywhere between 20-100k (this page was 40k). Opera has a pretty cool status bar that tells you exactly how big each site you open is.
Hmm, can't find the opera link synchronise feature. That's one of the best features of opera imho that I can have my bookmarks and start page synchronised across all my pcs.
My old k750i is not liking the interface that much though. bah.