You will not make more money because you learn Prolog. I'd approach it like a hobby. Some people like collecting stamps. Some like pondering esoteric programming languages. With things like Prolog, you are closer to Nature, so to speak. Or maybe more mathematical. Learning about a lot of "modern" technology is a lot like memorizing minutia about the semi-arbitrary choices that have been fossilized and accumulating cruft over time. YMMV.