What's a free or cheap text-to-speech program that is available for commercial use?
eSpeak is a brilliant opensource text to speech project. It's the default TTS software on many Linux distributations, but there's a Mac version available and it works great. There is also a project based on eSpeak called Mbrola which adds "singing" functionality.

Fwiw, I really don't see Apple suing you if you use the built-in TTS they provide.