New Yorkers paid an average of $1.73 per square foot in co-op maintenance in the fourth quarter of 2018, according to real estate appraisal firm Miller Samuel. So for an 800-square-foot apartment, the monthly maintenance bill was $1,384. But that number can fluctuate wildly, even within a co-op. Here, according to Brick Underground, are the 10 Commandments that govern the mysterious art of setting maintenance charges: