As with most divers ; you want to progress as far as you can in this hobby/sport or as far as your bank account will allow.. I initially started all my training with NAUI many years ago, and wanting to take it all the way, signed up for the Instructor course at my LDS, and was shocked at the cost of 3300.00 and the books alone were 1500.00. You have to repurchase all the student materials that you already have ( like they really changed anything in the last few years except the pictures ).. SSI was 550.00 for the course, and PADI is about the same..it's easy to see with prices like this why no one is doing Naui anymore and making the conversions to SSI/ SDI..