11-12 GT500s are significantly cheaper now than when the 13-14s launched. All 4 years have held steady or gained a smidge of value since the 2015 launched.. because there has been no GT500 officially announced. As far as the market is currently concerned, the GT500 is out of production.
The GT350 will no doubt get some 500 owners to trade em in because a serious road course car is what they are after. (Personally I think the GT350 is going to be better at road coursing than the Boss.. and I thought the Boss was a bad mofo!)
There wont be enough to sway the GT500 used market with just the GT350. That wont come until the next GT500 is announced. Im predicting a Twin Turbo Ecoboost V8 powering the next gen GT500. THAT will likely get the used 500's to drop in retail value. 11-12s should come down into the high 30's to low 40's. 2013-2014 will be a solid mid to high 40's as it really is an incredible performance car with supercar features. But still, significantly less than their new asking price.