.. my favorite LDS recently. They charge $5 for air, $10 for 32%, a flat $15 for O2 <= 40 cu ft, which I find very reasonable. He was saying that the up-charge for nitrox is basically a labor charge. They go out of their way to fill slowly, cool the tank, fill some more, cool the tank, top off, etc. So I end up with a full 3442 psi of 32% in my HP tanks when cold, a proper fill. I respect that, they're my favorite for a reason.
...There is another LDS ...They have a water-cooled filling/blending station, and filled my tank in about 15 minutes with 32%. I was happy enough, but by the time the tank cooled down it was down to 3200 psi in my shed....
I see a lot of variation in filling practices too. My closest LDS has a very cold water tank. They give 5 minute fills & my pressure usually increases a couple hundred pounds after I leave. My second closest shop fills in open air & takes closer to half an hour. I usually don't loose more than 50-75 psi after the fact. Prices are close to the same between the two shops.
Most shops around here bank at least two nitrox mixes as well as air. At least one local shop has a couple flavors of trimix on tap as well.
Prices around here vary with the quantity of fills you prepay in advance. For air, you might get $7 for one, $60 for 10, $125 for 25, & $450 for 100. Nitrox is probably 3-4 bucks more per fill. Those aren't actual current prices, but they are pretty close.
I have another shop about 45 minutes away that Is $15-20 cheaper on hydro tests, but about $10 more on nitrox fills.
I have one shop who charges by the tank & another that charges by the cubic foot. Guess who fills my 120's & guess who fills my pony tanks.