You are splitting hairs. My point is that markups for individual items elsewhere in the marketplace are frequently in excess of 400%. You can walk away from, say, a restaurant whose markup on food is too high for you, just as you can decline to buy nitrox if you don't want to pay the asking price. The dive ops do not have to justify their markups on individual pieces of their economic picture to you or to me, and they are not doing anything wrong by charging $10 for a nitrox tank. Take it or leave it.
I ate in an upscale restaurant the other night where I paid $36 for a dozen Gulf oysters. I ate a couple of nights later at a cheaper place where I paid $16 for a dozen of the same oysters (actually they gave me 15 because they were small). Am I going to go back to the first place and tell them that they overcharged me? Of course not; it was my choice to get them or not.
No argument there. As I previously said, I opted not to buy nitrox on my last visit because of the cost. Had it been more reasonable, I would probably have got the upgrade. If I'd done that, I'd have been happier, and the dive shop would have made some extra money (just not the 400% or whatever it works out to). In the end, a business has to decide what they market will bear. I don't think many Cozumel diveshops are losing customers over the price of nitrox. They're just selling less nitrox than they might otherwise sell. Maybe enough people buy it anyway that they don't think it would be beneficial to reduce the price.
I just figured it's probably not much extra effort to order 50 air and 20 nitrox (or whatever they need for that day) than it is to just order 70 air from merry-d. Maybe I'm wrong about the $100ish/year cost of a nitrox analyzer being significant to their bottom line.