I’ve had some experience in instrument building (harps duh). I do it not as a way of making money, really, but because building instruments is one of the last refuges of consummate craftsmanship in our culture. While all the other crap in our lives gets more removed from the human hand, there’s still an understanding that instruments come from craftsmen, not factories. The relationship between a musician and her instrument is intimate enough that they’re willing to invest the kind of money that near perfection costs. You get to work with gorgeous and incredibly expensive woods and push your skills to the limit.
You have to have to be something of an obsessive perfectionist to enjoy and succeed in instrument building, though. The market is full of examples of superlative craftsmanship, so mediocre instruments suffer badly by comparison. If the idea of working for days to get a finish just right turns you off, then it’s not for you. You also need a strong intuitive, if not academic, understanding of physics.
Building guitars and bowed instruments is different from building harps. Most instruments already have an established optimum design. Virtually all modern violins are knock-offs of Strads. Building these instruments comes down to trying for a perfect execution of those well-established standards. Harps, on the other hand, are still very much evolving, and no one design has achieved the status of being the optimum way of making a harp. That’s exciting, because the door is wide open for experimentation. All harpmakers are out there messing with very different ways of doing stuff, and every now and then someone hits on a really nice idea.
As for the “lucrative” part, I don’t know. Maybe it could be, with enough effort. Personally, I wouldn’t want to calculate how much I make on an hourly basis with harps. Yes, the buyer pays a lot of money, but each harp represents a good four months (at least) of virtually all my spare time. I’m not concerned enough with efficiency and I spend too much time relishing the process. Better not to count, in my case.
If I were to actually try to make a living at it and I lived in an area with enough harp teachers to provide a market, I think I’d build a stable of solid, good-sounding harps from modest materials that I’d rent out to harp students. Because of the cost of a good harp, most beginners will want to rent until they become dedicated enough to buy. Then you’d have a good chance of getting their orders for a nice custom harp down the road.