Cloud isn't about cost though. Cloud is about value. You can scale super easy when you inevitably need it (assuming you've "made it" anyway - whatever that means). You get a burst of new users, it's trivial to add additional nodes (again, assuming you've set up your infra to be easily scalable).

With own hardware, scaling is not as easy. You'll have to do a lot more around plumbing too. Networking, security, many other things that you'll have to address. Stuff that has already been solved for you.