It has been almost a decade since my Adv Nitrox course, but I am trying to figure this one out. You are not doing trimix and from what I remember of the the course spec you are mostly working around bottom mixes that are hypoxic at the surface and dealing with a ceiling on the dive profile. Even with the bottom mix in dual cylinders on my back, a descent/ascent mix on once side an a 50% mix for the last few stops on the other side I can't see how you were getting charged $800 for the mix unless you were getting completely hosed on everything that was not a standard 32 or 36 recreational nitrox mix.

OTOH you are completely correct about it being a deceptively expensive hobby, even if you take the "cheaper" route and go with semi-closed rebreathers or even OC.