The KR doesn't even need someone to prognosticate on the value. The car isn't some killer exotic, and it's not going to be the last fast Mustang. It's a factory Stang with a couple FRPP parts installed by Shelby out in Vegas that'll be faster than a C6 but slower than a Z06.
The only thing you have to decide is whether or not you just have to be one of the 1000 people that get the car. Looks great, has some cool CF bits, nice hood, better stance, much better grip, etc. Shelby won't be around forever, and this is a car that legitimately has his stamp of approval. If that's worth something to you, then go for it. Otherwise, buy a GT500 and spend a few thousand bucks.
As for pricing, some dealers are going to be in for a rude awakening. If they think the car is going to be $50k and they can tack on 30-40 grand for themselves, they're wrong. MSRP for the car will be higher than you think because Ford has learned it's lesson with the FGT and GT500. Think they regret pricing the GT at 139k only to watch dealerships get 200k? Does that mean dealers won't try and tack on the same amount? Of course not, but I think they'll find quickly they'll be screwing themselves trying not to be the last man standing when they realize buyers for a 120k bolt-on Mustang are few and far between...