Cop cars aren't collectively bought. For one its on a state by state basis, and then in some states individual PDs are responsible for their own vehicles, they just get a budget to buy and fix them.
That means they usually end up in an imbalanced power relationship of small 20-30 person local PD force approaching Ford / GM asking for their police cruisers.
Some states, of course, do collectively bargain their service vehicles like that, the USPS is probably the most prominent example of collective bargaining on a national scale - the post office bought 140,000 of them over 7 years for below consumer grade vehicle pricing. Even nowadays when they are reevaluating replacing their fleets it only averages out to about 41k per truck for a new model and that includes having to refit garages and post offices to accommodate them.
I'm also pretty sure there are licensing deals between US auto makers and various state bureaucracies to price control service vehicle costs, but its still never like a private automaker is getting an order for 80k cop cars from one entity.