In NY city, differentials for cert, BSN, etc have been in the contracts since at least the early 80s. New York has a longer history of contracts than other regions in the US, and each contract, as you know, builds on the one previous. It may be helpful and enlightening for you to get a hand on the contract of some NYC hospitals. This is not hard to do. All hospitals in Manhattan are unionized, minus one [New York Hospital which to be viable must adhere to the general pay scales established by the other, union hospitals and so follows their suit] and a copy of the contract is gladly sent with any request for employment there. In it, you can see all these diffs, and also the base salaries, as well as other aspects of contracts ever stregnthened by negotiations and new contract language including issues like floating and rotation, nocturnal parking priviledges, etc, etc etc.
Write to the recruitors of St Lukes/ Roosevelt, Mt Sinai, St Vincents, Colombia Presbyterian, Harlem Hospital, etc, etc, and request a contract with application material, and you will get one from each. [links to New York hospitals are found at http://www.noah-health.org/english/hospitals/nyhosp2.html#Manhattan]
This may be the sort of thing that will REALLY help your negotiation team.
GOOD LUCK and keep up the good work.