Hi Rick,
A1 Scuba charges $5 for air fills. That can be clean air if you have Nitrox clean tanks, or their standard gas for non-02 clean tanks. For Nitrox they are a bit pricey at $12 a fill last I checked.
This is typical for most Denver area shops. In the case of A1, if you go during retail hours there is Always someone who can do standard fills, but clean fills require Blender qualified staff, and they are not there at all hours.
Gander Mtn. is a good deal, however unless they are on your way somewhere, it's hardly worth saving a dollar to go out of the way to use them.
Odyssey likely has the cheapest Nitrox fills. I think they were $7 last I checked, but they are way north, and one has to weigh the time and gas cost vs. paying the more common $12 per tank for Nitrox from a shop that maybe more convenient.