NYC already has a built in COLA, it's called Juniority. If you can hold Captain, or Wide Body F/O, only in NYC, you are being paid more in NYC (COLA) than what you would be paid in ATL, because you can't hold that position (or pay) anywhere but NYC.

Now...LAX is a whole different can of worms, because they don't have as many positions available out there. If they had all the same equipment based there, as in NYC, and if they had to cover 3 airports (do they still cover only 2?) it might go as junior as NYC.