The Auction House has nothing to do with proper supply and demand or at least not directly.
I have mentioned this before, but the way to go about it is to have no price limits (sorry legendary painters and tuners) and open up the analytics to the users and not just have them running in the background to determine max buyouts.
When you select a car you should see all current offers on the same car and some sort of summary to suggest a reasonable price for the car. The latter could be a little graph of the sales prices of the last 100 cars. it doesn’t have to be complex and we don’t have to make it into a full stock market kinda thing, just at least have a bit of info and an actual free market.
In addition it might be interesting to include a fee for putting a car on there even if it doesn’t sell to encourage players to really think about a decent price for the car.
Basically do what WoW does and we get true supply/demand.
Sure, you could have a total cap of 20 million total still if you don’t want the system to completely run rampant, but don’t limit prices per car.
But this is never going to happen of course 