By my math, the bill for their app and database servers would be approaching $30,000 per month. That doesn't include storage costs, bandwidth, or any of the other aspects of their infrastructure.

That's crazy, if you ask me.

Is that calculation taking reserved instances in to account?

No, I don't think so. Latchkey did the same calculation, using on-demand prices and came up with $35k[1]

3rd reserved is roughly 48% cheaper than on-demand, so real hosting cost would be around $18,200 for those servers.

[1] http://news.ycombinator.com/item?id=3306394