Maybe this Answer it....

Market value of a TV Station is based on their revenue according to the market size. Most of the times the revenue or profits are based on high viewers ratings data. In many mayor markets, the networks provides an affiliation deal by offering stations network compensation. Compensation makes the difference between what a satin affiliate's revenue actually is and what it would be if all network spots have been sold locally factoring in the value of the network programs.  
Independent commercial spots costs depends mainly on absolute raitings. (total estimate audience) of the program in which the ads occur.
In other words market size ranking and viewers raitings goes hand in hand to determine a station market size value.