One of the problems with Viper values is that people looking to buy one and researching the forums come and read the owners themselves undervalue their own cars by undervaluing the vipers people are trying to sell. The majority of people that speak to me about my viper cannot believe that it was bought in 09 for 37k (2000 ACR with 14k miles). I now have 26,500 miles and that's basically considered high miles and should be sold for 14k. These cars are such low production vehicles (especially after how many have been totaled) yet the community really undervalues them. Interestingly, it is mostly people who have already sold their cars that seem to be so vehement in discussing their value.
Acura NSX cars were of similar MSRP; yet their values held better. Many have 90k+ miles and still sell well (don't know the particulars anymore). I think as more vipers are wrecked, and as the Gen V hits the streets, we will see the values of the Gen I/II increase in value. I would venture to say that the lowest values have already occurred and the prices are already increasing slightly. I'd say prices have increased an average of 3k from the prices from 2009-2012.
It's unfair to compare the values of high mileage NSX to a high mileage Viper. The viper market is full of very low mileage cars, which makes the high mileage cars less desirable--this isn't true for the NSX.
A fair comparison would be between a low mileage NSX to a low mileage viper, the price difference isn't much--especially when you consider that the NSX sold for more then $10k more than a GTS back in 96 (nearly $20k more than a RT).