Imagine BMW can make electronic engine control units for $10 each, and the spare part market will bear them charging $500 for them, so they do charge that much.
When I repair it saves me $500 but when the manufacturer repairs it only saves them $10.
What do you mean by "cheap enough for the manufacturer to just throw out"? The manufacturer generally has no need to fix the devices after the warranty period, they're not necessarily in the repair business. If the devices are designed not to be fixable and need to be replaced then you throw it out and buy a new one from them, earning them more profit than the repair would have. If their warranty defect rate is low enough, they have no need to be able to repair them themselves.
