O2 is not the big cost in making Nitrox. In my area a K bottle of O2 (highest price around) is about $24. Assuming you mix 32% all the time, that bottle will get you, ideally, about 18 fills. What's that, not even a buck and a half a fill for the O2?
A cheap(er) way to get Nitrox would be to fill your O2 cleaned tank with the proper amount of O2 at home, buy a small appropriate (modified E) filter stack, and then take both to the LDS for an air top-off. Cost of the O2 is marginal, the filter stack will be a few hundred, The air also is cheap. As someone has already said, since you'll be filtering air that is pretty clean to begin with, those filters will last a long time.
I highly advise taking a blending course before you attempt any of this. "Playing" with pure oxygen in your garage is not to be taken lightly, and the LDS will (should) take you seriously if you show them you have taken the course.
And of course, you have to decide whether or not you really want to hassle with all this to save how much money. What is your time worth?
Neil