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If the land is zoned for a service station, then an argument could certainly be made that fair compensation should be based upon its value as a commercial property, not an agricultural property. However, the cost of preparing to construct a service station that was never in fact built is not an expense for which the state will offer compensation, nor is the theoretical value that the land would have if the service station had actually been constructed -- if it's a vacant parcel, it will be valued as such.

If the landowners wish to try to pursue greater compensation, and have not yet done so, they should consider working with a real estate lawyer who handles eminent domain cases.