There's no questions about nitrox being better than air--
A question then based on what I would see as a faulty premise.
.... so I can't justify it no matter how much I prefer it.
Prefer or Need? If it is
needed for a specific dive profile, it is worth whatever they charge you for it.
The deep Oki wrecks (IIRC) were beyond EAN32 limits. Everything else worth seeing, calculated off against a repetitive dive schedule (3 a day, even on contiguous days), didn't seem that I personally "needed" it with my depth profiles and SI.
$10 is okay, but $15...Doing 3 dives twice a week would come out to almost $400 a month just in fills.
6 dives scheduled in a week? This is from your home base in Okinawa at your "local LDS". If a weeklong visitor came to an AI in the Southern Pacific, yes, he would certainly be offered a "weekly rate". I understand that your situation is individual and different.
You do have an advantage, though. Most of your local dive shop's retail customers do not make 6 dives in a week. Not even close. Look at their "package" prices, usually based on a one or two tank equation. Ask them for a bulk purchase price, then make them a counter-offer on a multi tank package deal. (Negotiations 101) this will work even better if they see you as a long term customer.
Until recently, only your local Okinawan
US Military (recreational)
concessions could offer nitrox due to Oki governmental red tape. I believe that now a very few of retail civilian "dive shop" operations are currently set up and licensed to do so. This means:
it's a seller's market. But, I do see that Bluefield Dive is offering it for $10. (not your choke point of $15) Refer back to "negotiations", as above.
What's everyone else's dividing line for choosing between air and nitrox?
In final sum, you have very little algebraic choice. Decide if, for whatever reason, that if you "need" nitrox, then shop around for the best pricing. If, in this equation, you come up with factor one being "No, I don't need nitrox", the formula results in:
0 x $ = _____.
Pretty easy.
If the
dive profile and the
repetitive dive schedule requires nitrox,
it is worth paying whatever for. Otherwise, breathe the cheap stuff.