There's no simple answer but here is some information that may get you started.
Calculating your estimated bandwidth used is actually fairly simple. All you need to do is:
>Estimate the average page size of your site. Put this size in KB. If you don't know, use Pingdom's Load Time test on a few pages and take the average.
>Multiply this time your average number of visitors per month.
In turn, multiply this times the average number of page views per visitor.
>If you need help with this, MarbleHost has a basic tool for doing this math and even converts the final answer into GB, making it useful for determining which account you should buy without having to carry the decimal.
It's important to note though that, to do this, you have to calculate for every site that is going to be on the server. If you run twelve domains on the same account, take the time to add up the visitors for all sites combined and the average
page size across all domains.