30GB would be far too much for a new site I think, unless it has download files or expects a HUGE amount of hits when its just starting.
Only a developed site would need around 30GB or one that uses a lot of images, download files etc.
You cant really give an overall figure which is good for any site, you just have to be individual for each site, every site would use a different amount.
You could have a small business site which is run in a small town that might use up to 5mb bandwidth/month or a large business which might have a few stores in large area which would use GBs of bandwidth.
It just depends on how many people visit a site and the size of each page.
Also, files available for download could use a lot of bandwidth, but again, this depends on how many people download the file..
Theres a lot to think about for how much bandwidth is needed for a site really