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...
Assuming $1.00 per 1,000 impressions, and taking their 2.07billion impressions/month figure:
Roughly 2,070,000,000 / 1,000 = $2,070,000 in ad revenue per month?
Wild guess.
