Because OP example is very simplistic and left on the table very important details, you would base 250M on a single machine? What about backups, obervability, how do you update that stack without bringing down everything ... Also this is napkin maths, this could be off by 10 or 100x which would change everything.
It's very simple to make a PoC on a very powerful machine, make it ready from production serving hunderd of millions of users is completely different.
