A good example of buying a 70 Duster done rather than building one

I see a bare shell sell for $3000.00 to $5000.00. A slant car for $6000.00 to $8000.00. What does it cost to do ur power train, paint and body and wheels and tires. Pretty soon ur working for nothing. Some guys even have to change frame rails or the trans frame. Also u see cars for $15,000.00 that just need to be assambled. Rarely are all the parts there. Kim