It can varry depending on how you want to scale it just like any business. not everything has to be crashed and tested to meet NHTSA standards, companies like SCG I recall do not or didn’t do crash tests, cars sold in limited quantities don’t have to be crashed long as they are sold under the 250 limit a year of which would mark them as a kit car manufacturer which I recall some former automotive company did years back to not get into trouble with the U.S goverment. As for companies like Pagani and Ghost Squadron vehicles, there’s been exemptions under show and display that let them bring the cars into the states without a crash test of said models. I know the egg did crash tests with the same reusable chassis which is actually really cool but most companies do not have that level of engineering and care because of our capitalistic approach.

TLDR: cost can be very low if you can start from scratch if you really wanted to out of your own garage post material marketing molding manufacturing like many other companies have done with tube frame kits such as DF Goblins and various other that just borrow other platforms: G35 vaydor as an example. however the higher you want to reach a level of standard the more it will cost an it gets expensive real quick to the point profitability becomes non existent as an actual car manufacturer unless you go mainstream and sell in mass which just means more and more money dumped into the company.

For those who don’t know, https://youtu.be/aGIYU2Xznb4?si=2h7Xo4_0cAqzcAQ7