I would agree with the waiting 15 years assessment. In the last few years the market has been flooded with manuscripts, but eventually those large caches will dry up and we'll see what the value really is.
If you take away Scott's (Abandoned Planet) liquidation I think we'd be looking at different prices today. Actually, that's what he did, bought a large collection and sat on it for 10 or 15 years, so I'm sure he made out well. I doubt that large buy was his only source, but it may have been the major part.
Of course in 15 years those of us still around may be communicating with smoke signals and spears, and the manuscripts won't seem quite as important. You never know. ;)