I generally don't work up there but it looks easy to spend $1,800 per square foot in the Lucida, the new green condo going up on 85th Street.
Co-ops will be cheaper (buyers generally prefer not having to go through a co-op board) and it looks like some contracts are going through at $1,400 a square foot, while it looks like some things are going begging at $1,100 per.
So it depends on what your definition of a "nice" apartment is. Other things being equal, you will pay more for a family-sized apartment (one with at least three bedrooms, or at least that potential), a killer kitchen, renovations that are already done, a more prestigious building, the blocks just off the park, and a condominium rather than a co-operative organization.
Hope that gives you some guidance -- and if you're going through an apartment hunt in New York, you might enjoy my book.
author, "Diary of a Real Estate Rookie"
Insider Real Estate Tips with a Twist of Humor: http://tinyurl.com/2ag28z