Developer wants to buy my primary residence

[Steven Helminen]

A developer currently has a ~8.5 acre parcel under contract that mine and 3 neighbors share a boundary with. He is interested in buying us out to develop independent living homes for senior citizens. How do I begin to assess what I should sell my property at if we begin to negotiate? He quoted $/acre as a metric he uses to assess deals. I am open to selling! 

I have not given him my selling price and would like him to name his price first. Any help direction would be much appreciated!