To be be honest the thing to affect Miata prices will be this. The end of the new Miata.
Something yo consider. Mazda shared the latest car with Fiat to keep the platform alive. The Fiat realty is not doing well and is a rare sight on the roads. This is a sign Fiat my not move ahead. If Mazda has no replacement for the Fiat this could be the end.
Also putting a battery large enough to make for a decent range could prove to large and heavy for a small light sports car to really survive well.
Here is the deal on the older models. #1 conditions and rare or special popular editions will climb. As conditions drop so will the prices.
One has to remember the Miata has sold in great numbers so there is a ton of them out there. Granted many have rusted in some areas but there are still many. Like a C5 Corvette supplies are still good. A Z06 can bring bigger money but an average C5 can still be a great deal at $12k.
Also other cars are all going up. Even 88 Fiero’s are pushing $20k today in good condition and T Tops.
The other problem is if the price goes up too much then this brings in other interesting sport models to buy.
I really think if the car is discontinued this will be what pushes the fans of the car to the used market and to higher prices as demand is the control of value.