I checked Heroku free tier, it is a toy of a server (512M). I'm from Java world, and that thing could barely launch HelloWorld.java.
However, a some $20 VPS (2G-4G) should be able to host a single app server + single DB good - which is enough until The Growth happens, and only at that point it is time to add another VPS to have a cluster of apps servers and migrate DB to a replicated setup. Hopefully there are already paying customers who will cover the costs.
That is, of course, if we're bootstrapping. If on the other hand, we're "serious" about it, then why not, let's throw some money at it, because we must be in a real business if we spend the money like that, right?