Well, not only that but most people don't know how to operate a 3D printer, nor how to customize the hand to fit the individual. So while the $42,000 hand includes the professional services involved with fitting the device, the 3D printed one depends on a charitable donation from the 3D printing operator to do the work. It won't be much but it's all probably going to be significantly higher ($400-$1,000) for the human being to operate and configure the 3D model for the individual.
I bet there are only $100 worth of aluminum, gold, silicon and acrylic in the $42,000 hand too.
This drives me crazy when people don't include the costs of labor. It would be like someone getting a car donated to them and saying "Wow cars only cost $200 for a title and registration! Why do people pay thousands of dollars for a car!?" Because someone gave you one for free!
I work in film production. I have a $50k camera that I rent out. On most productions your total rental per day will be about $2,000 a day. Now you could say that you could shoot a TV commercial for "only $2,000 in rentals!" But that ignores the fact that cameras don't operate on their own, lights don't just place themselves, actors should be paid for their skills, assistant directors need to keep production on schedule, locations need to be paid for the rights to use their property etc. So yeah it "only costs $2,000" as long as you ignore the $20,000 per day in crew costs for a small production.
People who say a film only costs $20,000 to make are either productions that somehow shot and finished in 1 day or else they're saying that their crew's time was worth nothing.
If I spent $10,000 on a 3D printer. I couldn't just open the box and push the "Give me a 3D Prosthetic Hand" button. I would either need to spend a good week or so learning how to print ($100 an hour * 40 hours = $4,000) or else spend $1,000 on a professional to setup and configure the scene/hand for me specifically. /Rant.