Very early in my career, I did quite a bit of sub-contract work, and it was fairly profitable. In order to succeed, you have to be able to charge the right hourly rate. Many prime contractors want you to have your own tools, vehicle, insurance, and bonds and licenses, yet want to pay you as if you were an hourly employee. This is a recipe for disaster.
You need to figure out the minimum salary that you want to earn and all of your costs (including licenses, bonds and insurance) to come up with your hourly rate. Keep in mind that you won't be billing out 100% of your available time, so your costs must be spread out over the actual number of hours that you can expect to reasonably bill (typically 60-80% of your time if you are lucky). If you do your calculations right, you may be shocked at just how high of an hourly rate that you will need to charge to make a profit.
You also need to expect peaks and valleys in your revenue stream. When things start to get slow, the use of subcontractors is one of the first things to be cut, as companies want to give what work is available to their own employees.