I fail to see how spending $3500 on a $4k watch make any sense, given the fact that the case might still be rep....Which is why I would never do a "uber-franken" 16610 sub build unless a whack of cheap gen parts just fell in my lap.
However, spending $7k on a 6538 big crown sub build can be justified since that is only 1/10 the cost of a genuine equivalent.
But, this is a hobby, and most hobbies just don't make economic sense. How many vintage car builds do you see at Barrett-Jackson where the cost to build the "resto-mod" is significantly higher than the sale price? Happens all the time. Part of the fun is in the building and part-hunting process whether its a budget build using a cartel case or a complete genenstein.
I personally enjoy the fact that I have to contact several vintage parts dealers, or scour ebay daily looking for the perfect part. I think if there was a super watch parts supermarket where I could find every part I ever wanted, the fun of this hobby would be lost. It would just be too easy.
As for cost breakdown, i think that's somewhat irrelevant.. how would you price a rare insert you might have had in a drawer for 20 years? at cost or current market value? If the latter, how is that determined? I think for any build, only a complete list of parts (gen and rep) are required along with the services/mods performed to the case or any other part. Then it's up to the buyer to do their homework and evaluate if the asking price is fair or not.