Depends on the condition, leather or cloth, with speaker on the headrest, color combination, and etc....
I got my early style miata seats for $80, but the driver seat has hole on it, so I end up switching the passenger side one and put on my driver side and modify the slider and the seat mount on the zed. Finally, put a miata leather seat cover on it and call it done......
With a decent leather miata seat, they run for upper $300. Then again, what type of miata seat are we talking about, that's the key question. Good luck with your hunt......
Ed