I don't know the details but from my observations they are:
- Office space
- Legal fees
- Events (demo days, etc., are catered)
- Salaries
- Weekly food costs
I actually thought the operating costs would be slightly higher. The office is a comfortable space and the food is great(IMO, others might disagree :) ), the events are always well done, you'd expect the legal fees to be high, not to mention how much time everyone at YC puts in.
Are those all per startup? I assume office space, legal fees and salaries refer to YC and not its startups (e.g. YC's legal fees, not the startups' legal fees). Food would be per startup of course.
It's my understanding YC does the incorporation paperwork for the vast majority of the startups it funds. So there is a per-startup legal fee for that.