To a certain type of person, it would be fun and challenging and satisfying.

Worth it? Well, the main cost is time and effort. Its also highly helpful to have a pile of old parts you can make use of. If you have to buy much, the costs are going to catch up to you almost immediately. Even as little as a tube here, a tire there, some housings here, will make it difficult to do without it turning into problem.

That said, I encourage you to give it a try. I think its good for you (if you are that type that enjoys the work as therapy), and also good for the community if you are making cheap and decent bikes available. In my case, I started out playing around with my own frankenbikes. In the course of snapping up a couple of parts bikes, I started a small supply of spares. Then I flipped a couple of bikes. A few more parts bikes, a few more bikes for sale. And now, I have a small supply of usable wheels, not-quite-worn-out tires, tubes, salvageable housing, a box of derailleurs, a box of brakes, and the like. This vastly changes how I size up a bike in the trash. I.e., does it have a nice frame but tacoed wheels? I can decide if its worth devoting a spare wheelset on it.

jim