The last major party convention held in New York City was the Democratic National Convention in 1992. Security costs were $6.6 million.

Source: Campbell, James T. "Convention '92: Security expected to be easier here." The Houston Chronicle, July 16, 1992, p. B5.

This year the Republican National Convention is expected to spend an estimated $65 million on security.

Source: Hedges, Michael. "2004 Republican Convention New York..." The Houston Chronicle, August 29, 2004, p. A14.

That's about 9.85 times as much as the 1992 DNC, not counting inflation.