That is a factor but not the biggest one. It's about demand. In the US we have a so called medical "free market" so the cost is as much as the market will allow. So, if you are missing a leg, how much is a prosthetic worth to you? You'll find that unlike with say, a stick of gum, the answer will vary dramatically with the key differentiators being how much the person has and whether they have loved ones they must care for who they value more than themselves. Now, abstract that cost from real people and put it on collectives with billions of dollars to spend (insurance companies) and why wouldn't you charge six figures for a prosthetic?
For $100,000 there are thousands of people who could engineer a prosthetic that can be customized with just a few hours labor. So the $100,000 cost is spread among all of them and the customization part amounts to a few bucks in plastic and under $1000 labor and that is at doctor labor prices and not lab tech prices.
But these products require FDA approval. So that is going to cost another $250k. Which is great for you if you have that money. It means that you get legal immunity at the end. It means little to no competition. It means you won't have to worry about actually improving your device anytime soon. It means you can charge ridiculous prices which are easy to justify, you can point to the need for FDA approval, you can point to the importance of making the device safe for medical use, etc. People will pay anything they can afford and since the bill goes to the insurance company, people will sign off on literally any figure. So it's really just a question of charging as much as the insurance company can afford.
The prosthetics end up costing the manufacturer maybe $2000 customized in the end with everything included and that figure goes down over time but they keep on charging $100,000 a pop because they can.