I struggle with the idea of cost, too. Just recently I did a quick cost scenario of what I would like to have in my '73 in the end. Just throwing quick numbers at things and assuming buying kits and not building anything much, I was in the upper $20K range if I included the purchase price of the car.
But if I had my '15 Challenger R/T and my G3/T56 swapped Duster at the same time, and the Duster had AC, I think I would drive the Duster more. And performance wise I am sure the Duster would kick the Challenger's tail. Add that if I am into this car for $10-12K after I get the G3/T56 swap done, I don't believe I would ever find a V8, RWD, 6 speed anything for that price. So while I might end up spending the same or more in the end, I don't have to spend it all up front. But it does mean that I have to wait to enjoy it as compared to buying something and just driving it.
I should add to be completely honest, I still keep thinking an S197 Mustang with a 4.6 would still be the easy button. Good thing my wife is adamant that I will never own a newer Mustang. :D