PADI has largest market share of industry. Logical then if looking for employment there will be greater demand for PADI instructors. I had NASDS, then SSI now PADI instructor ratings. Taught for ten years with NASDS and SSI. For me , in my experience, PADI had most opportunities for employment and highest pay. There are over 7 PADI facilities in a 15 mile distance from my home here in New York. There are no NAUI, SDI, or SSI facilities nearby. Nearest NAUI facility is in Brooklyn, about a 1 hour + drive away. I do not know if it true about cost of becoming an instructor with other agency's , but why go with least expensive if there is next to no chance of finding work with them? I suggest going to facilities you may want to associate with, ask about chance of getting assigned classes there , what agency they work with, and then choose what you want to do. If you are willing to invest your own $$$$$ and get enough gear to run your own classes, rent pools, etc, then you may want to go with an agency that allows independents. We have independent instructors use our pool at a greatly reduced rate if their students purchase mask, snorkel, fins, boots from the facility.