Rent Office New York

For my entrepreneurship class I do a business plan. For my business, I need to rent a small office space in New York. Where can I look online to find out how much it would cost to rent offices in New York? Or where do I find listings of offices in NYC online?

everything I see they want you to sign up to get prices. But craigslist is priced
http://newyork.craigslist.org/mnh/off/