Laellee, in my experience, it seems to depend on the market. In the militaria market, there seem to be plenty of people willing to to dupe someone else to unload a piece they now know is fake. That is mostly among the older crowd, it seems, but it seems to happen there more then anywhere else. With the internet, that has gotten much harder, thank goodness.
In the art world, people will spend tens of thousands of dollars to try to have their paintings or prints deemed authentic, even if they have good reason to expect they have a fake. This can go on for years, in the hopes that they can recoup their losses. A lot of pieces disappear into the woodwork for years, decades even, only to show up again, in the hopes that the next generation of authorities will re-examine the piece, and call them good.
Prop collecting is still a fairly recent phenomenon, and because it has such a narrow nostalgia window, holding onto it for a few decades is probably not a good move. Look at what happened with one of the Kurtz/Bies sabers. If I remember right, it sold for $200k and change, to Paul Allen, or at least his museum. 5 odd years later, it show up again, sells for $400k to Ripley's. (Correction: Not the same lightsaber, but both were from Kurtz, according to SethS.) Smart move if you have something of questionable or no history...
Right now, it seems that the market consensus is that a lightsaber from someone who is in a position to have likely acquired a production made saber is at least $150-250k. Regardless of how "original" the parts or, or if it was even screen used. Can you imagine what one that had rock solid provenance would go for?
After a bit more research, and some thinking, I'd honestly expect that an absolutely authentic saber would sell for more then the Ripley's, but not by a tremendous amount. That end of the market is pretty small, I imagine, and has been burned before. They're probably wary... if not, they should be!
Edited for correction. Thanks SethS!