$400 dollars for a deep diving course sounds extremely expensive to me. At my LDS specialty courses are usually around 99-125 a piece. Besides isn’t deep diving a must for your Advanced cert. I know for my Advanced OW deep and navigation were mandatory and the other two specialties were up to. By the way my AOW costs only 250 and that was four specialties; drysuit, deep, nav, and search and recovery. I do think though if you are planning to do deep dives and cavern those are must courses. I feel there’s some courses that will be more beneficial to you depending on the conditions that you dive in. The more you dive, the more comfortable you will be, and the better diver you will become.