There is no real easy answer to this question but you should be able to make a reasonably decent estimate based on the size of your site and your expected traffic. The size of each web page on your site is very easy to determine prior to your publishing it. Take a very simple example. You have a one page business card site that has some basic information about your site and a few graphics. You add up the size of the home page as it is published on your server, then add together any other file that is linked to your site, for example graphic files.

For the sake of this example lets say that your total with graphics for your home page is 650KB (100 Kilo Bytes). Now you need to estimate the number of visitors that will load your page each day. Remember, it's not just unique visitors, it can be anyone that returnes to view your site multiple times in one day. Lets estimate that to be 1000 views of the home page per day. With the exception of a little overhead, your total transfer over the web is 650MB per day.  The lowest bandwidth rate we can provide with any reliability is 256kbit/sec which equals approximately 2.7GB/day. So for your simple site you have more than enough bandwidth.

The example above is a good example to show how to simply calculate your bandwidth requirements. But why would a small one page site need a dedicated server. It wouldn't. For the most part your site will have GB of data transfer/day on a dedicated server.