If a niche magazine publishes an excerpt of a soon-to-be published book in that niche, would the writer normally be paid? Or is the writer typically not paid seeing as the excerpt serves as good publicity for the book? Is there an industry standard on this situation?
Yes. It's called first serial rights. And yes, you get paid. Even if it's not much. "Good publicity" is a feature article or a review. It's not an excerpt from the book.
Sharpie magazine editors are always looking for good content. Don't let them skate on paying you by telling you they are doing you a favor by publishing it.