Parts prices are a big chunk of the bill; depending on the quality, the barrels and pistons can be as much as $3500. The parts will run $2500 - $5000, then add in labor. It's been 25+ years since I've done a VW engine and seem to recall that was around 25hrs, the Porsche has a few more pieces and tighter clearances would be at least that. This doesn't include any machine work. So at a minimum figure $2500 in parts, $2500 labor and throw in $500 of machine work...........presto $5500.
Now back to the general topic: unlike most racers I keep every receipt because I want to know the total for the reasons the topic states. For my Datsun 1200 everything was used or pick a part, $250 car, $300 paint and supplies done in the garage (10 footer), A15 engine and 5 speed tranny $500, Machine shop bill $1000, engine parts $750, Keihin carbs $350, Header $100, upgraded cylinder head $300, LSD 4.11 rear end $700, wheels and tires $500, Steering wheel $75, suspension & brake upgrade $1200. The interior is now striped but when it was a street car we had installed a $200 set of Mustang GT seats and found a $50 rear seat at pick a part. The rest of the interior bits can be had for about $250............................$6000 for a Datsun 1200 and that doesn't include labor. The best one in the country, a twin carb JDM spec coupe went for $6500. Note I offset most of the cost on my car by selling off other parts I got with the various deals I did.
I too believe there is a set minimum price and I easily seeing it being 20K if you're paying for labor. Where it's cheaper to restore a common car is the labor, I can assemble a Datsun A-series motor in an afternoon, compare that to a Ferrari V12 or Cosworth DFV, the same would hold true body work on a Facel Vega vs a Chevy Vega.
If you were trying to make a living flipping Spridgets you'd need to carefully layout how things were done, which brings us back to enthusiasts. I also am into 70's Japanese bikes mostly Motocross, I've seen guys replacing everything and going over the top. If the engine is within spec leave is alone, the paint on these bikes was never that good, often the plastics can be refurbished. A perfect example would be the airbox on my YZ125C; one of the mounting pieces that you can only see when the air filter is out was broken, I used epoxy to fix it where as a friends bike got a new airbox...........to the tune of $150.00,
I could go on even more but you get the idea.
Now if you love the car why worry about spending 20K of you;re disposable income on a car. Well I may be the only old car nut who thinks so but there is nothing worse than some hole of cash out in the garage that is worth a third of what you put into it, if it's value is near what's been spent friends and family are far less likely to give you grief..........and no matter how much people say I don't care what people say it does detract from the joy of ownership................a classic should never be a burden, they are meant to be a source of joy.
Tom