Ok, I am going to Chine in here... My PADI DM Course was 575+DM Crew Pack, which is about 350 + Diving Expensives (1/2 Charter Rate, however if i am in the lake helping, it was completely free... Free Air and weights). However, with other organization that i am with, I pay 75/year for my DM Certification, IT and Instructors pay 125/year + insurance. PADI You have DM/AI insurance or DM/AI Assistant Only Insurance, however the difference is only 50$ or so.
SEI, Does not require me to have Insurance as a DM, as I am covered under the store policy when I am assisting an instructor with Class and Dives. However, especially here in the US, it is worth it to have professional liability insurance. Additionally, I ALSO Have DAN Dive Accident Insurance, again, for the cost vs. what you could spend for a diving accident treatment, I dont complain about the 110 for my membership and Diving Insurance