Some shops are alot more expensive than I am for the DM course. My price is higher than $600, I was just commenting that it was the lowest I had seen.
Keep in mind that all of my instruction is totally private/semi-private for families friends taking the course together. OW courses in your local dive shop are not private.
My price for OW also includes everything needed (equipment, PIC, books, marine park fees). I also include two boat dives for the last two OW dives.
For DM, they also get one-on-one instruction, and boat diving almost daily for 3 to 4 weeks during the course. Books are not included and of course as a DM candidate, they are required to have their own gear. As someone else mentioned above, Aqua Safari charges $850 for the DM course. I know of another shop that charges$950. My rate is considerably lower than that and is in line with most shops on the island.