^ agree with Zhorno, This will differ depending on so many factors.
Perhaps, those who own z's, who live in your area, who are of your age or similar, who have similar mods, etc etc crashed into their garage, and the formulae the insurers use is still appreciating the cost of the garage because of this.
I would venture, that if you had a different car, were in a different area, did a different job, you would see different results.