I have a Protank 2, and was able to find the air control ring (version1) at one of my local vape stores for $4.
Admittedly, the v1 air ring isn't quite as open as the V2 that comes on the aerotanks, or the newest variety with multiple holes in a line... but it was in front of me, and it was $4.
Money well spent. I can still use the same coils that I've got, and the singles are cheaper and easier to rebuild (if that is your thing). And you already have the Protank 2... most of the tank parts are the same from pro to aero, with the excepton of machining for a longer coil.
Cost benefit time... Do you want a backup tank? Can you get a cheap air ring? Do you want to be able to say you're rocking dual coil?
IMO, the add-on for the protank is cheaper and at least 90% as good as buying a whole new rig.