Subscription based license is wrong in my opinion. Lot of money investment is required. You can charge one time license and keep very little subscription fees for upgradation. After all once you developed the whole application, there very little investment is required to keep running the ball.
Also in countries like india and aftica, $ payment gets multiplied by our currency and already costly applications like you gets costlier and invites the interest of hackers and crackers.
May be you can keep the costing same as $ in our currencies also.
So we can purchase your application one time.