If you are considering renovating a NYC brownstone in Harlem, Brooklyn or anywhere in the city, take a look at this article from Brick Underground. Granted it is a little bit of an adverticle but it does give you the basics as far as timelines, potential costs and pitfalls.
Not that long ago, I came into contact with a contractor who renovates Harlem brownstones and he told me that $175 per square foot would be a very middle of the road renovation. A property that I sold in Hamilton Heights achieved a full renovation for $400,000 and it had about 3200 square foot but she was a seasoned investor and was very hands on in the project.
Another investor that I am dealing with from out of state hired contractors and each succeeding one had to redo the mistakes of the previous one and that ended up being costly. So, I would say if you are seasoned investor you are obviously going to spend less and if you are rookie might be better to pay more and have the process managed by someone that you can trust.