PortableApps.com on sourceforge reports they had 943799 downloads last month. Popular files include Firefox portable (20mb) and chrome portable (2mb). Assuming the average download is half way between those (10mb) you're looking at about 480,000 GB of downloads a year. If you store that on S3 you'll be looking at about $46,800 of bandwidth charges a year.
Quite true. The total is quite a bit higher as some popular apps like GIMP (136,000 downloads in the last month) are 64MB and July and August are our slowest months. Additionally, apps like Inkscape are hosted on their own SourceForge project and LibreOffice is hosted by The Document Foundation (not included in the above numbers). Then there's all the freeware we host elsewhere that doesn't show in the SourceForge stats (which we pay the bandwidth on). All told, I think we're easily exceeding a petabyte a year. I used to actually keep those stats, maybe I should look into it again.
Surprisingly expensive business.