I have a bit of insight into this, so I'll tread lightly. You are 100% wrong on the margins on these things being huge, in actuality the good mfg's, UWLD, Light Monkey etc have surprisingly low margins because the cost is very high to produce. You want to see real insane margins? Check out the recreational side with 100% markup as standard from the dealers, not including the markup already there so the MFG makes their money. Same with DPV's, the reason you never see stuff like this on sale is because there is no room to mark anything down.
You say li-ion like a macbook, correct, however these packs are significantly larger, typically 2x-3x and have to be custom made. They now also have to be certified which is very expensive. It costs on average $5k to certify a new battery pack in this market, that isn't cheap. The actual cost of most of the battery packs to the mfg is somewhere around $300, give or take, so write that down.
Scale- the big light mfg's, Light Monkey, Dive Rite, UWLD, etc may sell a grand total, combined, globally, of 3000 lights per year. IF they're all lucky. That doesn't leave a lot of room for mass manufacture. Except for final assembly, everything in a macbook pro is completely automated in terms of the boards being soldered, etc etc. These lights have to be hand assembled including soldering on all of the connectors, etc etc. There is no way to automate this, especially considering the scale. Man hours of assembly per light are high call it 4 for easy math, and since they can't afford to have full time staff building them, they have to pay themselves, so call it $100 in man hours of assembly at $25/hour, write it down.
Machining. All of the stuff has to be milled because economy of scale says that moulding them doesn't make sense. Shearwater FINALLY hit the economy of scale where it made more sense to mould instead of mill and they released the Perdix. The Petrel is 4 years old, and the company is 12, with a far higher demand, and employ somewhere around 30 full time staff. UWLD has 1, Light Monkey has 3 I think? The machine time alone per product is somewhere around $100
Raw Materials, very high quality materials going into these lights to depth rate them as deep as they are. Aluminum, custom glass optics *at least in UWLD, everyone else is using off the shelf*, delrin rods, etc. $200 or so for the charger and misc raw materials.
Ish call that $700 for a run of the mill standard light. MFG has to make some money to cover the costs of running a business and hopefully making profit to warrant the effort etc so call it $900, and then the dealers have to make some money to warrant selling them so call it $1100. Right around tthe price of the Dive Rite LX25, the base UWLD, Light Monkey 21w etc. Prices go up for various improvements in terms of bigger batteries, more powerful light output etc etc all of which have to be properly designed.
I know the price models and yes they make money, but not a huge amount of it, and they deserve to make money because otherwise they wouldn't have devoted their careers to do it. You want cheap primaries, go to Asia where they are using inferior raw materials and cheap labor. Big Blue, Ano, DRiS brand etc etc but if you want the real deal you have to be willing to pay for it. Apple has factories dedicated to nothing but mass produced products and I promise you that they make a LOT more money per product than companies like UWLD, but they also have to invest in those factories etc etc. There is simply not enough divers in the world that need canister lights to be able to have the costs come down on stuff like this. High quality products are expensive no matter what industry you're in, especially when they have to be tested to things like 200m depth limits, and be durable enough to be abused so you don't break anything etc etc