What about offering file downloads

Simple web pages don't use up much bandwidth, the same can't be said about downloading files. If you plan on allowing people to download music files, pdf files, flash files, or video files, you can eat up bandwidth in a hurry even with a relatively small number of people visiting your website. If your website offers an adobe .pdf file that is 1 megabyte in size, with one gigabyte of bandwidth, you will only be able to offer one thousand downloads. This does not including the html needed to get people to download the pdf in the first place.

Video files eat up even more space. A one megabyte video file represents only seconds of video. For a half hour presentation, your video file could easily be over two hundred megabytes in size! At that size, one gigabyte would only allow you to offer five downloads to your visitors. What you plan to do with your website most definitely will affect how much monthly bandwidth you anticipate needing.