I can't understand why they charge for the air. I'm from Brazil and all the gas stations have free air. On top of that, almost all of them use digital air stations where you put the pressure # you want and the machine will beep once the configured value is reached. I never saw one of those digital stations here in Canada... they are awesome!
I have my own compressor, a cheap one (45/50 bucks), but you have to remember they are not totally "free" as mentioned above. Most of the times you have to keep the engine on when you are using it and they may take 3-5 minutes per tire to reach the right pressure.