I've been making money online with SaaS for several years.
Don't try to invent anything new, that is too difficult. Just find something that exists and people need and do it better or more focused, and then market it. You will easily make money. How much? That depends on the service that you choose and how good you are marketing it.
Edit: to clarify, I'm a developer, so starting costs were close to none, but now I make enough money to have employees