" I was told that becoming a DM might subsidize my diving expenses" very misleading and often used to entice new divers into becoming DM students. Before you get to DM even did they tell you how much DM will cost you outside of the course itself? Do you own a full set of gear? You should. What about all the trips to the pool and dive sites that your training will require?
DM's unfortunately rarely make enough to subsidize their diving. And this is partly due to DM's themselves who choose to work for free and devalue the rating for others. Why buy the milk when you can get the cow for free right? That's how shops look at it. Being a DM may allow you to write off some expenses but in order to do that you have to show some kind of income from it. Ask the shop if they plan on hiring you and what the rate of compensation will be when they do? Ask them what kind of gear discount you will get and how often. How much will you get for doing pool sessions and checkout dives as an assistant?
And will you be required to do other things like work in the shop or rinsing and hauling gear while learning to be a DM? None of which is required by the way but shops seem to think it's ok to use DM candidates as free labor justifying it by saying those are DM duties. And how many classes will you get to intern if they are very busy? How many of their prior DMC's are now employed , not used but actually employed, by them and making some kind of money that justifies their statement?
Personally a DM, IMO, needs not only more than 50 dives to start but should have experience in salt and fresh water, cold, low vis, and be a damn good underwater navigator. I also think they should be required to take some type of tech class, even just a good intro to tech, before starting DM.