4.2.3. Objective Market Evidence; Conjectural and Speculative Evidence.For compensation to be “just, not merely to the individual whose property is taken, but to the public which is to pay for it[,]” its measure must be objective.258 The determination of market value must therefore take into account all considerations that might fairly be brought forward and reasonably be given substantial weight in bargaining between buyer and seller.259 But the appraiser must disregard any special value to the owner “who may not want to part with his land because of its special adaptability to his own use” as well as any special value to the government because of the government’s needs or the property’s “peculiar fitness” for the government’s purposes.260 Only “value transferable from one owner to another . . . has an external validity which makes it a fair measure of public obligation to compensate the loss incurred by an owner as a result of the taking of his property for public use.”261 As a result, “loss to the owner of nontransferable values deriving from his unique need for property or idiosyncratic attachment to it, like loss due to an exercise of the police power, is properly treated as part of the burden of common citizenship.”262 Similarly, “the Fifth Amendment allows the owner only the fair market value of his property; it does not guarantee him a return of his investment.”263 

Moreover, just compensation cannot be based on mere speculation or conjecture. As the Supreme Court stated in Olson v. United States: 

Elements affecting value that depend upon events or combinations of occurrences which, while within the realm of possibility, are not fairly shown to be reasonably probable, should be excluded from consideration, for that would be to allow mere speculation and conjecture to become a guide for the ascertainment of value—a thing to be condemned in business transactions as well as in judicial ascertainment of truth.264 100