The cost of hardware drops when your a power buyer and I would presume DigitalOcean is in a position to receive pretty decent discounts on hardware given their position in the market in just 24 months. Having seed money and financial backing just deepens the discounts and resource availability.
Any company with a large enough deployment and continuous purchases from a supplier is bound to get specialized pricing which converts to lower costs per physical server deployment.
While a rather broad example, if one server costs an everyday buyer $12,000 to build, DO may be in a position to build the same for $6-$8,000 which is a rather substantial reduction in costs, thus they're able to recover their investment quicker, even with lower costs per month to the end-user.
If you take the number of Droplets that are said to be deployed right now, and if that is a current production number, and you just take the base cost of $5 a month x Production, that's a rather hefty $500k+ per month. Of course, that discounts anyone use anything other than the $5 a month VPS's, so I would say that number of much higher.
Of course, I'm sure Moisey and the rest of the team isn't going to divulge exact numbers, but it's not too hard to see that they are indeed making money. As long as they keep up what they're doing now and as long as Mosiey sticks around, I think you'll see DO in operation 6 years from now, giving them the 8 year advantage as well :-).