Correct me if I am wrong, but doesn't the gas mark gear entry specify the cost for additional tanks? I believe it was listed as 40 nuyen each, one hour capacity.
I would question why you would choose both, as the respirator only provides a plus Rating to tests to resist, while a gas mask is immunity. Unless you're up to your eyeballs with Tyrant GM Inc, you should just go for the gas mask to be honest.