Hmm...none of those cars are even close to the Viper in terms of performance or driving excitement or exclusivity or appearance or....you get the idea.
It's true that the GTS has a much higher base price than either the 911 or the GTR, but they are priced pretty closely to the SRT Viper (base car) and similarly equipped in their basic forms. Especially the Porsche, the MSRP might be mid $80s, but Porsche charges extra for EVERYTHING (oh, you want four wheels another $3000, seatbelts $1000, windshield wipers $1500, etc). The GTR has an MSRP of $96K so it is basically the same price.
Furthermore, you can't get a car with anywhere near 640 bhp unless you buy one of the last ZR1s (which splits the difference between the base SRT and GTS price wise) or step up to a McLaren MP4-12C, both of which are merely close, but not equal.
Personally, I was pretty impressed with the price. For an extensively updated car that now has a carbon fiber body, leather dash, and fully revised suspension, and more power when the previous model was already ~$91K, the current price seems reasonable.
The thing that most impresses me though is the fact that the Viper remains such a connected driving experience compared to all of those other cars. It is still a driver's car (per the magazine first drives and VOI posts that I read), not a computer car that adjust your inputs. The GTR will secretly head out in the middle of the night and take drives without you (even in the left seat you are basically a passenger who gives directions), it's not so involved. One of my best friends had a '97 GTS for many years and I got to drive it quite a bit. It was MUCH, MUCH more fun than my C5 corvette, not just because it was significantly faster, but because you were rewarded for your hard work and you had to be mentally present for the whole drive. It felt special and exotic in a way that a Corvette can't match. I haven't driven a ZR1, but this same friend now has a 2009 Z06 and while the Z06 is definitely faster than the '97 GTS was, it is not as interesting to drive unless the car is near it's limits on the track or on a canyon road.
Anyway, long-winded post, but I bet that you will get a lot more for your money with a Gen V Viper than nearly any other sports car, even if you do spend >$100K.
Of course we all want different things out of our cars.
-Nick