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.
You're probably right as I calculated with expensive instances. Also, when I made my estimate I was guessing at image storage costs, forgetting that the images are coming from image sharing sites.
Many/most page views on Reddit don't have any ads. Promoted stories only appear on story lists, not individual story pages, and don't appear 100% of the time (the space is also used to promote random new submissions). The graphical slot in the sidebar is almost 100% non-paid in-house ads.
I've always wondered why they don't contract out with other ad networks when they cannot fill the ad content themselves. Say for example their self serve ad can't fill the page request why not put in a google text ad link on the right side where the banner is? That to me seems like a straightforward way to massively increase revenues.
They dont do it because they really care about the user. Just sticking up random google ads isnt going to make anybody happier and with an internet savy crowd like reddit ad clicks are likley to be low. Sure very targeted ads like the ones that self-serve currently delivers work because its redditors advetising to redditors.