Vacant Lot In Danger of Possible Eminent Domain Worth The Risk?

[John Bell]

I currently have a verbal offer on a half acre lot in the heart of the downtown area (one street over from the current courthouse) of a small town in the path of progress. The local government has announced they will be building a county courthouse, district attorneys office and jail next to the current court house. They have acquired the land for the courthouse and jail already but they have yet to get the land for the new district attorneys office. They have expressed they want the new buildings all near the current courthouse. My concern is as soon as I buy this lot the government will seize it thru eminent domain. I'm hoping to build a small strip center and lease to local business that will benefit the new government employees. (coffee, lunch, bail bonds, etc.)  

Does anyone know how fair market value is calculated for eminent domain? If they take the land will I get at least what I put into it or take a loss? If I build will it deter them from taking the land? Another vacant lot is closer to the current courthouse.