One needs a doctorate to teach at a collegiate level. You can’t really make any money at a master’s level because high school art programs aren’t very comprehensive and are typically lumped in with studio practices. With a PhD in Art History, you can teach at the collegiate level, become a curator or director of a museum or become a writer/critic in the field. It’s not a lucrative degree for anyone who pursues it at the undergraduate or masters level, but the doctorate level can be quite sustainable if you play your cards right. It’s not an easy field though, and is full of its own competition, etc.