An early Texas proposal, later rejected as possibly too speculative, inequitable, and complex, linked the price to repair costs and insurance company data for replacement values. EMVR programs' tendency to attract the least driven vehicles with the fewest remaining miles is especially severe when the price paid is low. Such pricing policies may cause other problems. Older cars are not as fuel efficient, or as safe, as the newer cars likely to replace them. MERC prices also won't reflect the social benefits of lower levels of pollutants already below legal limits, or the probable benefits of evening out emissions by time of day.
Society also benefits from replacement lags, and when some non-replacement reduces traffic.
Those external benefits justify government subsidization of accelerated vehicle retirement. Without MERC sources, pollution sources are few enough in some areas to cause monopoly behavior, including higher prices and the hoarding of credits to limit competition. In some places, the MERCs could make emission offset banking and inter-firm offset trading viable. The spatial redistribution of emissions may or may not be desirable. It depends on the location of the ambient air quality monitors, and the spatial distribution of people.