As the title asks, how much in general would it be sensible to mark up car prices from the base material costs?
As per Der Bayer, a basic car should cost twice the cost and premium stuff three times the cost.
So:
Basic car costing $10k sell for $20k
Luxury Saloon costing $20k sell for $60k
Supercar costing $100k sell for $400k
Hope this helps 
While the above is a good guideline you also want to take a good look at how you would theoretically produce this car, of course.
For example, the Willow Calavante costs 14.6 grand to produce, but is sold at $36,000 in the SP model. Meanwhile, the V Series costs nearly 16 grand but starts at $33,000. Why is this logical? Because the Calavante is a 2-seater sporty coupe, with a big engine producing over 400 horsepower, its place in the market is exponentially lower than that of the V Series, which is a comfortable, safe, economic family saloon car. Thus for the V Series, the profit per car can be lower, as the theoretically and logically higher sales should be able to compensate.
Ultimately, a good starting place for a high-production, mainstream automobile IS twice what it costs to produce, but then you should honestly probably tweak the price from there, either going up or down depending on how well you plan to sell the car.
If you ever wanted to take a detailed look at how CNSpots’ explanation might figure for changing markets, just take a look at the various Car Tycoon Challenges that were previously run by Der Bayer to simulate exactly this kind of thing. It factored in competition, market share, engineering and manufacturing costs and capacity versus price and profit per unit etc.