"market value"

means the estimated amount for which the property should exchange on the valuation date between a willing buyer and a willing seller in an arm’s length transaction after proper marketing and where the parties had each acted knowledgeably, prudently and without compulsion: Provided that in determining market value for purposes of section 12(1)(a), prices paid by the State for any acquisition of property must be excluded: Provided further that in the event that no other credible data is available, prices paid by the State for any acquisition of property may be considered;