You are correct, MonoTouch and MonoDroid are 400 USD each. No need to wonder anymore!
As for Unity, it is a great platform, very mature and a million developers have adopted it as their platform of choice. Unity is powered by the same Mono engine that powers MonoTouch and Mono for Android.
Use the tool that is most appropriate for your needs. For instance, Microsoft recently published the iPhone Kinectimals game on the Apple AppStore and it was powered by Unity3D. Not surprising, since at least 10% of the top sellers on the Apple AppStore are powered by Unity3D.