Compressed air is definitely cheaper than any other gas you could use, but you can't liquefy air at room temperature, at least not reasonably. As result, a can of compressed air would have very low capacity compared to a gas which can be liquefied at room temp.

A gas like propane for your BBQ liquefies at room temp, and once it's liquid you can put a lot of it into a cylinder without increasing the pressure.

That's why compressed air is not used in the spray cans.