We host thousands of Discourse sites on maybe ~40-50 servers at the moment. So the ratio isn’t that far off, but it is true the .net framework is extremely efficient in ways that Ruby + Rails is … uhhh… not? Postgres has gotten quite competitive with SQL Server in the last decade, as well, so that’s good news, particularly in the “if you have to ask how much the licensing of SQL Server is gonna cost, you can’t afford it” department 
Our yearly software licensing bill is basically $0, though we do give back when we can. I can assure you Stack Overflow’s yearly software licensing bill is more on the order of $100,000 … perhaps a whole lot more. It wouldn’t surprise me if it was closer to $250k.