I really dont understand. Im deploy rails webapps all the time with pretty large postgres backends.

You could totally spin up multiple digital ocean instances to get a much cheaper infrastructure. I usually use one for load balancing, 2+ for running the app, and usually one for the backend postgres. You could also get away with using something other than digital ocean for the postgres.

But for 6000 users, depending on the app. I can probably get away with like $30 a month with this setup. Including the database.

Total setup time might be a day the first time. But it scales pretty well, costs significantly less.