I think $10-$12 for Nitrox is pretty typical, but for me it is just one more incremental cost. $60-$85 for a 2-tank, a tip for the DM, a tip for the Captain, $20.00 for Nitrox. For my wife and I that would mean around $1200.00 per week to do just 2 dives a day with Nitrox. It adds up fast and has driven me to much more shore diving. At least in Bonaire it is easy to get free Nitrox and since I can do 3-4 dives a day pretty easily and cheaply there, that is a good thing.
These days, I can hardly afford more than 2 dives a day and maybe one or two night dives in a week on Cozumel, so nitrox isn't much of an issue there and I have cut back on it when I go.
I think the larger operators should purchase their own cylinders, compressor and related equipment and fill their own tanks. Beyond the basic air station setup cost, I believe it is only a few thousand bucks more to be able to do Nitrox as well. I would think the larger operations, like Christi's, Aldora, etc., could save enough in tank rentals, fill fees, gas and employee time to pick up and drop off tanks to recoup that investment in a couple years at most and then begin turning a profit on it. They could even offer nitrox as a no cost or low cost upgrade and that would be a draw for customers. And, I would think the people who want nitrox tend to dive more and are a desireable customer base.
I guess this would be bad for the small op, though.
Of course I have never run a dive operation and may be full of you know what. Isn't the internet great?