I'm with ^ that guy. Run, do not walk, away from this POS! I've seen Miatas in pretty nice shape for $2500-3500, and that thing is not in good shape. The rust is in a pretty bad spot, not a replaceable panel, and he just painted over it instead of fixing it, which could give you an idea of the level of care he's put into everything else on the car.
Replacing the top will run you $300. Unless you're going to use this as a track-only car, and are willing to let it rust away, you'll probably want to have it painted, too. With his rattle-can job on there, you're going to end up paying a lot more for the prep-time (mucho sando) it'll take.
Offer him $800-$1000. If he says no, walk away. The condition is bad, the mileage is ok but not amazing, there's no reason to rush into this car. Keep your eye out, spend a few more bucks, you'll find much more car for not that much more money.