As mentioned by me and others. Dives should be guided, or at least supervised. So, what would you think is a fair salary for the guide and the costs of his dive gear? And the person filling the Nitrox? We can agree they are two professionals doing a skilled job? What about the compressor costs such as energy, the place to run it, filters, oil, maintenance and overhauls. The transport of the tanks? Maintenance of the tanks? What about taxes and permit costs for the operation?
Yes, many people are willing to work for much less than what would be fair. Because they are over their heads in debts after more than years without any income from their dive operations. Please do your own math. And calculate what would be a fair price to pay. 30 USD per dive is not covering the costs for small scale operators where it can happen that the guide have to take only one or two guests diving at the same time.
Why should a 'Dive Pro' or 'serious diver' be charged less than any other customer? I am an excellent eater, one of the best I dare to say, but never paid less because of that in any restaurant ;-).