Given how vertically integrated Tesla is, this is the right approach. Include a factor for time on capital equipment (number which can be produced per year per machine, cost of machine, cheap cost of capital, wear on machines, depreciation over lifetime) and you'd probably be good (maybe $3k total for the part?)

If it's a commodity item (like most electric motors, but maybe not the one in the Tesla), with a competitive market, you might be able to estimate cost by looking at open market pricing, but for something like SpaceX, that clearly doesn't work.

Tesla is closer to SpaceX in thinking than it is to most car companies, especially startup car companies.