Usually 1,000 ad impressions is around $1.00 (varies greatly though, can be lower like $0.15c and higher like $4.00+).

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.

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.