Back when exactions were still new, two professors wrote a law review article about them, in which they ventured a prediction of how high these exactions (or impact fees, as they are sometimes referred to) would or could go. (see Heyman & Gilhool, The Constitutionality of Imposing of Increased Community Costs on New Suburban Residents Through Subdivision Exactions, 73 Yale L. J. 119, 1156 (1987)). There, they estimated that $500 per unit would be the uppermost amount that would be charged, and voiced the naïve belief that “legislative and judicial pressure will tend to require the establishment of a reasonable ceiling.”
Fast forward to today. The New York Times reports that a Montgomery County, Maryland, developer estimates that “he pays $50,000 to $60,000 a unit in impact fees.” See Emily Badger, Developers, the Indispensable Antiheroes of an Urban Drama,” N.Y. Times, Aug. 3, 2019, at p. B3.
Apart from that gem, we suggest you read that NY Times article because it provides a good insight into the problems that face developers when they set out to build housing.