LOTS of topics on this. Here's a great buying guide to get you going:
http://www.hondatuningmagazine.com/includes/htup_0810_acura_nsx_buyers_guide/viewall.html
Some advice that I have:
1. Try to reach out to people local to you and see if they attend cars & coffee. Buy them a cup of coffee and ask them to talk you through all the stuff about their car, you will learn a LOT from various owners.
2. There are 4 basic "groups" of NSXs: 1991-1994 coupe, 1995-1996, 1997-2001, 2002-2005. Pricing tends to fall in line for those groups with variances for color & mileage.
3. Unless the history of the car is well known (and even if it isn't), get a PPI from a recommended NSX mechanic. Not just an Acura dealership, but someone who actually knows the ins and outs of a NSX. Again, ask around on the local forums for insight. There are things that their experienced eye will catch that could introduce additional costs ranging anywhere from $1000-$15,000 in additional expenses when you buy the car.
4. Price is a highly debated topic and as you have noticed varies widely. While the NSX is a reliable exotic, it is an exotic and that means that buying a "cheap" NSX can end up being a very expensive NSX in the long run.
Good luck in your search
*edit* regarding salvages, I suspect that these are marketable cars and that the cost/reward curve for fixing one up is good enough for people to buy a wrecked one and then turn it around. Check some of the build threads on Prime for the work done by Shawn and MacNinja in restoring damaged cars, they are both artists and produce great fix-r-uppers.
