One year and half ago, it was calculated and then confirmed by an admin[1] that the monthly cost was around 22K/month, or 270K/year. jedberg added that they were projecting to be around 350K/year by the end of 2010.

Supposing that the cost increased linearly with the number of users (which sounds like a bad hypothesis, but is a start), the cost at the end of 2011 could be around 1M/year... That's impressive, but nowhere near the 300K/month proposed by rdouble.

So I would say that the monthy cost of reddit's infrastructure is around 90K. Which is really impressive.

1: http://www.reddit.com/r/blog/comments/ctz7c/your_gold_dollar...