We’ll probably never know the exact costs for sure, but I bet we can get a pretty good estimate...
I have some Ford Motor Company pricing tables for the 1967 model year (see the first three attachments), which show the Wholesale Price, Suggested List Price, and Suggested Retail Price for all three body styles, and all Regular Production Options as well.
I also have a background in manufacturing, and in a typical manufacturing scenario, the Wholesale Price is often somewhere around the halfway point between the actual Manufacturing Cost and the List Price (or sometimes it’s the halfway point between the actual Manufacturing Cost and the Retail Price). Not saying this is exactly how Ford did it, but it's probably close.
Since we know the Wholesale, Suggested List, and Suggested Retail Prices from the Ford pricing tables, we can then calculate a few rough estimates of the actual Manufacturing Cost...
So for example (see my final attachment), if we look at the pricing for a nicely optioned 1967 Mustang Fastback, we see that the Suggested Retail Price was $3,863.96, the Suggest List Price was $3,641.20, and the Wholesale Price to Dealers was $2,901.86 (not including transportation costs. BTW - this is based on a car I actually owned a long time ago).
If we assume that the Wholesale Price ($2,901.86) is the halfway point between the Suggested Retail Price ($3,863.96) and the Manufacturing Cost; then the Manufacturing Cost would be equal to: $2,901.86 - ($3,863.96 - $2,901.86) = $1,939.76
If we assume that the Wholesale Price ($2,901.86) is the halfway point between the Suggested List Price ($3,641.20) and the Manufacturing Cost; then the Manufacturing Cost would be equal to: $2,901.86 - ($3,641.20 - $2,901.86) = $2,162.52
And if we take the average of the highest estimate ($2,162.52) and the lowest estimate ($1,939.76), we end up at an average estimated Manufacturing Cost of $2,051.14.
So it probably only cost Ford about $2,000 to make that beautiful ’67 Mustang Fastback that was sold way back when at Lynn Kirby Ford in Portland, Oregon for $3,863.96 (plus another $135.20 in transportation charges for a grand total of $3,999.16)...