Ask HN: How do a website's costs scale? Where does money go?

Costs don't scale with hits per day, they scale with peak simultaneous load. Assuming that your content site is not getting most of its traffic from being on the front page of Reddit on a daily basis, peak simultaneous load corresponding to 100, 1k, 5k, 10k, and 100k hits a day is so low as to be laughable in 2011. There should be no difference in hosting costs.

You could theoretically end up paying more going from 1k to 100k if you have per-event-priced analytics, but that is also going to get swamped by your programmers' coffee budget, so who cares.

Sometimes API calls can be a meaningful expense, depending on what you're using and how.

Weird resource usage can sometimes be a problem if you don't watch it. Infinite recursive operations, brute-force algorithms being called over and over per user, stuff like that.

Advertising will take whatever you throw at it.

Personnel costs are likely to be the dominant factor for most web projects until you start talking serious, serious scale.