Besides the annual membership fees, the yearly insurance, the cost to stay current with training......Be prepared to spend money being a DM, not making money! I'm talking about your average landlocked DM or one who does not work a charter boat or is in a position to make customer tips.
You'll be told what a great discount you'll get and all the free air you can have. That's great up to a point. But after you have bought your new set of gear and computer, that discount won't count for much. You'll more than likely only get that discount from the dive shop you are working with, not all dive shops you go to. And all that free air will be used in a class situation anyway.
Most dive shops will schedule you for classes every weekend if you let them. Yes, at first it's a blast. But every single weekend, either pool time or OW checkouts and other classes, will really take a toll on your personal life, not to mention you seldom have room for fun diving on your own. If you wear your own gear during pool sessions, you'll be replacing alot of that gear yearly because the pool chlorine will literally eat it, especially anything with neoprene. Even with a discount, that's more money out of your pocket. Always try to use rental gear in the pool.
For those checkout weekends....in this area, until a new scuba park opened close by, we had to drive about 2 hours away (and many shops in the area still go here) which required a hotel stay, eating out, etc. for the weekend. That adds up to a minimum of $100 per weekend. Some instructors would buy our dinner on Sat. night but not even the dive shop would cover our travel or room expenses. And don't count on student tips, the concept never occurs to them and it's not mentioned during their class.
The first year Beast and I worked as DM's it cost us about $1500 each, that's not counting the cost of the class, insurance, or gear. On the other hand, it's the most rewarding thing you'll ever do for yourself, which make it worth everything. Just don't make the mistake alot of us did and work every weekend. You'll get burned out really quick that way. Take at least one weekend a month and dive for you, whether that means your local mudhole with your friends or a quick trip offshore...just don't neglect yourself in the process.