There is no limit when we discuss about the possible earnings that a person may get in creating niche sites as well as authority sites. It is because the income will only be limited to the money, connections, effort and the time you spend on the website/s you intend to have.
If you put in more effort on the websites, success will also be more. Definitely, more success means more money. A lot of time, money, promotion and research should be invested in creating a niche or an authority site. When you do so, you will also learn in the process.
Even as you try to create a website of your interest or for your friend, you have to know a lot of things that are connected with the creation of good websites. They are things like online legal issues, copyrights, techniques and methods and online language terminology and a lot more.