This is directly correlated with the servers and services you are utilizing. If you are using an expensive bandwidth provider, then the prices will go up, and vice versa. On average however, they tend to be low, however, you can have a standard price for bandwidth, that you would expect from each GB, and sell the bandwidth at a lower price, if the resources available are underused. This will drive up sales and increase usage.
Once usage has reached a certain point, then the host can then raise the prices to the bandwidth to a more appropriate level.
So to ask what the average price is, is to ask a very vague question, as the price will vary wildly among hosts for the reasons specified above, and therefore the average will have very little value when evaluating a single host.
Hope this helps.