Is there a good source for estimating the per unit cost breakdown for a classic GA plane like the Cessna 172?
You can find an estimate in the Roskam book "Airplane Design Part VIII"

Which is partial on google books:
https://books.google.nl/books?id=GIHHFkd829cC&amp;printsec=frontcover&amp;dq=roskam+part+VIII&amp;hl=en&amp;sa=X&amp;redir_esc=y#v=onepage&amp;q=roskam%20part%20VIII&amp;f=false

However, most Engineering Universities may have a copy of this series in their library. Most of the data you want to know, companies try their best to keep them private as it is information the competition would like to know.
In general most aircraft programs are based on an fixed set of sold units. If you know the cost of the individual units (aircraft) and the amount of units needed to sell to reach the break even point then you have a fairly good estimate over the program cost.