Stackoverflow famously ran on a single, although quite beefy server PC for a long time (not any longer, but not for performance reasons AFAIK).

I think it's a good data point to have to scale your workload to stackoverflow's, and reconsider the hardware costs.

(Obviously horizontal scaling has its place, but if it's that variably scalable, maybe there are better solutions, e.g. a single bigger instance -- often times even for cheaper)