Ask NH: how would you calculate traffic costs for your startup?

[jlla]

Say we are launching a new genius web-project which is doomed to become a new Facebook and Google simultaneously and rule the world. But what about the few firsts month after launching: are there any kind of practical advice how to correctly estimate hosting/traffic costs? What are the most common errors, under/over-estimations here? Should we use Amazon Cloud or not and why? Are there any statistics available, e.g. what Dropbox, Groupon, Netflix traffic/resource usage, what do they pay for it, how it evolved in time from the beginning? Do you have your own good/bad experience in calculating future resources costs for you startup?