I had almost the same conversation with my fiancee. Fast forward nine months, she loves her moissanite ring. I think what changed her mind was actually seeing a moissanite — it is very obviously not "fake," and the stone's fire is amazing.

Incidentally, rather than pitch it as "the ones you buy are created in a lab and are cheap," I'd go for something more along the lines of "they grow them through a natural process, kinda like pearls." It's not scientific, but it's true enough.