Even in other industries it is common that spare parts and consumables have a very high margin (while the initial purchase has a much smaller margin or in some cases is even subsidised).

The most well known example is probably printer ink/toner. (Razors is another often quoted example.) But this applies to car parts too. I needed a new small plastic clip to my Dacia. I was quoted 100 SEK (about 10 USD) for that. I 3D printed a sturdier version that will last longer for less than 5 SEK in materials (less than 0.5 USD).

From that you can estimate the approximate margins many companies have for spare parts. Of course being able to prevent cheaper third party parts will seem enticing if they want to maximise shareholder value. And this is why we need regulation.