Well, my 302 with all the machining, port and polish, aluminum heads, mustang intake system, Power steering pump, injectors,etc. cost me $6,500 or so in work and parts.
Now you can buy virtually the same thing from summit racing for $5,500 with a warrenty. I wish I had gone that route, but you learn from your mistakes.
And on buying a new one.... Now drivetrain wise, you save a bunch of money if you replaced everything with brandnew components, it may cost $9,000-$10,000 or more.
But it doesn't include a renewed warranty, fresh, rustless sheet metal, new rubber mounts, brakelines, better stereo, stress cracks, endless things that wear out over time.
I've always wanted to buy an early 70's ford truck for $1,000 and put $25,000 worth of work into it. So it'll be brand new, better than factory, but less the modern technology.
It could be done, I guess. But you would have to think and plan long and hard before you would go through with it.