My LDS charged me $100.00 for the PADI course, even at that price it was a waste of my time and my money. Didn't learn anything I didn't already know and I've used nitrox 3 times in the last 10 years. I haven't found any benefit to using nitrox for me.
My advise is to make sure you'll get something out of the cert before you pay. Ask yourself:
Can I exceed the NDLs with my tank? If like me you can't then nitrox will not give you more bottom time. If yes you have reason 1 for using nitrox.
Do you do multiple dives per day? If yes than you could benefit from nitrox. If no strike 2
Do you dive deep? The deeper you dive the less O2 in the blend.
When I used nitrox to dive the U853 at 120FSW I used 28% nitrox considering nitrox costs double compared to air I didn't getting much of what I paid for, O2. I was using doubles so I actually did get a little more bottom time, the second dive was also a little longer. However for the money involved it wasn't worth it. I also had to get my tanks O2 cleaned at $60.00 per tank, because my LDS blends they don't bank.
I'll just go on using air like I have since 1968.
Very surprised that you didn't use Nitrox much. But I suppose it also depends on how many repeat days diving you do, and the working depth.
The second time I went to Scapa Flow for a weeks diving I was Nitrox qualified, as was my buddy. Very unusual at the time (early 90's). Nitrox was only available from one skipper only (not ours). We used to nip down to his boat early, get out 3l cylinders filled with Nitrox 50, and use this on the (air) decompression stops each day. This massively reduced our fatigue, especially towards the end of the week.
The following year, we had Nitrox clean Twins, and had Nitrox each day, that massively reduced our decompression penalty.
Now, if we go to Scapa, we make sure everyone is Nitrox qualified. It's safer, more enjoyable, and reduces our decompression penalties.
That said, Scapa is perfect Nitrox diving.
Capital wrecks between 25m and 42m. Block ships are generally shallower than 20m.
It's two dives a day, capital ship in the morning, block ship in the afternoon. Temperate water diving, 5 days back to back. Decompression diving is the norm.
If on twins, we generally put a rich mix in for the afternoon dive (36%), then air top for the following morning to ensure a safe PO2. That keeps the cost down. Nitrox is blended, so cylinders need to be O2 clean.
Because I've been diving a CCR since early 2000's, I only now buy Nitrox for bailout, or when on Holiday and OC diving.