UCV bears (almost) absolutely no relation to the selling price of land. It is a system used by State and Local Governments for the rating value of property ie what they basis of council rates.
UCV is a statistical determination method of establishing land values having taken into account all sales in an area then adjusting (theoretically) for the development component, scarcity factors etc.
Refer to the recent court case of Maurici v NSW Govt (Land Tax Commissioner) whereby the plaintiff won the case due to the fact that the valuation did not take into account the lack of vacant/unimproved blocks in a typical inner city suburb of Sydney.
