$5-10k seems to be the standard for a signing bonus, but I'd err closer to the $10k for New York. As much as I think NYC realtors are a useless, overpaid class of people, it's difficult to attack the NY market without one, and they are egregiously expensive: 8.3-20% of first-year rent, usually 15.
twenty thousand dollars! ha, ha... just kidding! (only not really)