But it's not a targa - it's a cab... I mean a hardtop
I had a discussion with a well-known Porsche fabricator about this subject some time ago. He said that he felt a targa was a better base for a track car, due to the additional stiffening. That being said, he also bought one of my fiberglass tops for his race car.
Alas, you're probably right about $(parts) > $(whole car)