EMK...
I do not know what it takes to build a B$B but I do have a very good handle on what it takes to build a 302/FFR. I have seen several websites where guys have accounted for most everything and been a low as $19,000. There are several turn-key companies that offer a complete car and priced for much less than $50,000 plus. A few of their links are listed below.
http://www.427cobras.com/options.html
http://www.erareplicas.com/427/427sc.htm
http://www.uniquemotorcars.com/Price_sheet6_2004.html