There is Google Video, where they host videos for download by end users, Google Mail, Google Earth (which probably takes up a substantial amount too, given the live feed for users zooming in and out of the world), Google Pages, which hosts user-created pages, Google Blogger (need we say more? One of the more well known blog hosts in the Internet world), Google Web Accelerator (which proxy-fetches pages), Google News (which crawls news services for new information), and lastly, Google Search crawlers. All these are only some of the more bandwidth intensive applications Google has running on its servers. How about their AdSense network, which is serving advertisements on websites all over the world? From all these, I hazard that Google probably has at least 15 data-centres around the globe, with tons of fiber-optic connections to the world. Plenty of back-up backbone for fail-over scenarios! Assume one data-centre hosts 3 10 Gbps pipes... That would be at least 450 Gbps flowing through the pipes simultaneously! (But all that is a wild stab in the dark. I am sure Google uses much more bandwidth than that!)
regardless of whatever their absurd bandwidth number happens to be, I can pretty much guarantee you the bandwidth isn't expensive at all compared to the processing power and traditional server resources required. bandwidth isn't that expensive really. I personally have use almost 20TB a month on my servers, and my expenses are fairly minimal.
i read in an article most of their "servers" don't even have cases. just taped on hard drives all over the place.