Painting cars is also a process that scales. If you want to do a great job on one car everything you need can be had for less than $3000 (inflatable spray booth, large air compressor, a paint gun, and lots of sand paper). A good job can be done for less (a cheap air compressor and paint outside can still get good results), and most people who want to paint a car already have most of the tools so the cost goes down. The above setup will also handle a small auto body shop that is painting a couple cars per month, and you can upgrade equipment as needed (I'd start with a spray building instead of an inflatable booth).

At the scale of Tesla the millions of dollars on a fully automated setup is cheap because amortize the cost of so many cars