I don't know what company you are using, but in my experience, most shared hosts that I've gone with give me unlimited bandwidth per month. Years ago, this would have made no sense, but today, it seems like common practice. The logic is this:
- They can fit more customers on one box/VM than they could a couple years ago
- More sites are built using database intensive CMSs, so if they have, say 20 GB of traffic a month, their site should lag enough for them to upgrade to our VPS hosting
- By assigning a number or cap to bandwidth offered per month (like they have in your case), they can get people that are excited about their product to wonder if that number is high enough for their awesome product's site and maybe up sell them to an unlimited VPS or dedicated box
My suggestion to you is this: Look at other companies or plans. If you're hitting that much traffic per month constantly (which is a very good thing!), it makes no sense for you to overpay for their hosting if their just going to overcharge you. Thousands of hosting companies offer shared hosting with unlimited storage and bandwidth at probably the same price that you're paying right now.
This may not be the answer you're looking for (I hate suggesting to dump everything that is working right now for something new), but this is a dollars and cents issue. If you had a cell phone plan with X number of text messages and you're a huge texting teen, would you rather continue paying the overage charge for the texts or go for the unlimited plan and save money?