The GT is $80,000 more because it is $80,000 better. Performance is very close, but the GT is the better looking, more refined, and more bespoke (not all the switchgear is from another car) It also wont depreciate like a fat man falling out of an airplane like the Z06 already has began. (which is good new for the used market) Using the price logic doesn't make since because the GT is actually cheaper than its main rivals (Gallardo, 360, and 430)
I never once said it wasn't worth $80k more, simpy you cannot make a very convincing arguement on how cheap it is to modify a car that already costs way more than the other.