Obviously this depends a lot on the kind of usage patterns your application provides, but in my experience you should be able to comfortably run a 30k _daily_ users (the total n. of registered users could be 10x-100x that, depends) web app on a single $200-300/mo AWS machine (with hourly payments, you can get a better rate by prepaying). Get a bunch of cores and plenty of memory and put both your web app and the DB server on the same box for ultimate simplicity. Postpone scaling efforts for as long as humanly possible, probably until PMF and accelerating growth.
With reasonably "low level" tooling such as Java/Clojure/Haskell/whatever and a properly configured Postgres instance you should be able to go quite far. You're very unlikely to be CPU-challenged in the web app (again, no idea what your web app is going to be doing, so it's just a guess), most of the memory and CPU will be consumed by your database server caching and running queries. You should be able to handle a good 500-1000 db transactions / sec without much hassle.
IMHO most of the challenge will be making something that 10k people will want to use daily, not actually being able to scale to that many users.