also consider the pretty high cost of getting started. in some places (like where i am) there isn't access to rent tools you need to do a quality job. so you may have to prepare to buy some pretty expensive tamps, saws, laser etc lord knows you'll eventually need a skid and excavator.
I know a few companies that make really good money doing hardscape. But they specialize in it. I think once you try and do a job here and there you loose the efficiency and knowledge required to perform this service. I believe it is a skilled trade within the industry. A lot can go wrong if you don't know what you are doing.
It was easier to get started in this business twenty or thirty yrs ago. In my area you would now be competing with no less than 200 hardscapers.
Money earned is equal to time and effort you put into it. Plus a multitude of factors as already mentioned above.