You can use a site like Dollartimes to convert automation's prices. Basically, determine your modern sale price for the car, because Automation's currency system is that of the modern era, not 60's/70's/80's/90's. It's not comparable to those times. Then you use Dollartimes to convert your chosen price to the appropriate year.
One problem with a method some people here use, multiplying -cost here- by prod. units doesn't realistically work. Not because it's untrue in the slightest, but because because it's not as simple as that. Prod. Units is NOT man hours like it used to be, you also have to take into account how well equipped your manufacturer is.. As I believe it has been mentioned by the devs, tech pool effects your production costs and prod. units required to manufacturer ----, meaning realistically, sandbox modes Costs + Prod. Units calculations are not really comparable to an equipped manufacturer. For example, if you have 5 tech pool in, say, bottom end parts, you can turn quality up to 5 in said bottom end parts and get all of the benefits you'd normally get without taking a major hit in cost or productivity units, something that is NOT calculated in sandbox.
TL;DR prod. unit costs math is not particularly accurate except if your company is bog-standard with average tech ability in every category, really, it's up to you to think about how well-equipped your fictional manufacturer is, and then determine a viable cost based on the statistics the game gives you.
