I did not look into the details of the NOx emission credits trading program, so I don't know whether it could even be applied to cars.
I created the thread because I did want to point out that there is an officially accepted price tag for NOx emissions, and it is surprisingly low (a few hundred dollars per ton, which covers a million TDI miles) compared to the cost of even the most minor inspection, modification or repair on our cars.
The total cost for half a million cars is of course still in the hundred million dollar range. (Earl, there is an error in your math: 300*500,000 is 150 million, so 300*482,000 must be less than that.)