I don't know about the cost of the upgrade but I do personally love the two tone interior. I had a black interior convertible in a hot climate, and the seats would get so hot that I had to put the top up even for a minor errand in and out of a store. The two tone is not only classy looking but it does not heat up like the black leather.

I bought my car used, so there was no mark up for the interior, but when I looked at the new xfires at my local dealers, none had the two tone and I was sold that look.

Sorry if I didn't answer your questions...I just thought I'd offer my personal opinion, for whatever it's worth.