Also, there is the databases access that eats bandwidth, among other things. If the site has a lot of images, so the number of characters left to type (in your theory) are already falling down... but it's enough.
To take an exemple from a website that really does exist, a mIRC website with 1500-2000 unique visitors a day (few images if not of the screenshots section... but there are more than a thousand files available to download), it eats 30gb of bandwidth a month but it's growing every month, so you should be ok with 50gb for your site.
