I don't think prices are falling all that much. For about the last 3 years, we've been at a "floor" of about $20k - $25k for an average to good condition early coupe (91-94). I think we are going to see a divergence in the early coupe market in the coming years. Some of these early (and plentiful) NSXs will drop below $20k. These will be the cars that fall into the following categories:
1. Salvage/Rebuilt
2. Rough, abused and no maintenence (generally younger 3rd or 4th owners who cannot truly afford the car)
3. Hacked up with body kits, repaints, "car-show" type interiors, etc.
In contrast to the above categories, which are becoming more common as the early cars become more accessable, fully-maintained early coupes that are kept in OEM or close-to-OEM condition are getting rarer. This leads me to believe that such cars will begin to increase in price as time goes by.