Here's some anecdata from someone serving 1.5 million monthly users with a realtime (chat etc.) web app. Disregarding the file serving functionality, this is the entire infrastructure:

- The frontend is running on a 30 bucks/mo VPS with cycles and RAM to spare. - There's 2 DB servers for 5 bucks / mo for redundancy.

This thing would scale pretty linearly in theory, so for 10 million users just add like 4 servers.

So about 200 bucks / mo to run your average startup's web app at 10 mil users.

Alternatively write bad software and throw 50k/mo at AWS I guess.