If this is the only iPhone app you will ever develop, and you also have zero interest in developing Mac applications, ever, then MonoTouch is probably worth the cost.
If you think you'll ever develop more iPhone apps, or will ever want to do some Mac native development, it's probably worth it to learn Objective-C and the associated frameworks. Plus, if you're the type of programmer that enjoys learning new things, it's a fun new paradigm to study.
