If you're looking for good grip over long life and you're not going for the ultimate fastest time, why not pick up a set of Toyo RA-1s? They'll last for a very long time on a lightweight car.
Aim Tire has Miata sizes for good prices. Full tread 205/50-15s for $128.50, 225/45-15s for $136. FWIW, they have 205/50-15 R888s for $135 each as well.