Having put 42,000 miles in 3 years on my 97 GTS, I agree with Wayne. Anyone with a 96 Viper with only 400 miles really doesn't deserve to own it.
That said, in answer to your question "What should I expect to pay for a 96 with only 400 miles?", my answer is "Too Much!" Having participated in a 96-97 GTS search with a couple of friends, it is my experience that the owners of low-mileage cars expect an unrealistic amount of money for their cars. If I were buying a 96 or 97, I'd be looking for a clean car with somewhere between 7000 and 15000 miles and a transferable extended factory warranty (good for 7 yrs/50000 miles) that I could purchase in the low to mid $50's. That's purchasing a car from a private owner; dealers will be considerably higher (at least +$5K). If you do the work and look around, you can find excellent cars at that price range. (You can also find not-so-excellent cars at that price range.) Obviously, buying a car from someone local or close by has its advantages -- it's up to you to put a value on that.
Enlist the help of someone in your local VCA region, if you have questions about Vipers in general. And definitely have a Viper Tech or some knowledgable mechanic check your potential purchase out before you consummate the deal.