Most listings are in the $50's per square foot per year. The Chappaqua submarket is seeing a a boost in rates. In the last 3 months, the median rate increased $58 per sqft per year.
In the last 90 days, 1 listings have been made available, which means spaces most likley are going fast. You'll want to be prepared to tour spaces as quick as possible. About 200% of the available space in the Chappaqua are subleases, so you might be able to snag a great spot on a short term. There aren't many listings below 2,000 square feet available, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 750 sqft. Most buildings in this area were built in 1972 and are Class B.