system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] How monetarily valuable would a Lightsaber be in the Star Wars universe?

By rarity and tech alone a single lightsaber should hold significant monetary value.  

Significant cultural value

Significant Religious value

Significant technological value

Significant collectors value

It's a very useful tool and weapon

at the height of the Jedi there were on the order of 10k jedi.  You could assume many dual wielded etc so at it's maximum there would probably be 20k lightsabers in existence at their height if you consider stored or artifact sabers. 

20K of an item as significant as a Jedi's lightsaber is an extremely small number if you consider the breadth of the SW universe.  You would assume many collectors, bad guys, good guys, warriors etc would do whatever it took to get their hands on one. 

On top of this lightsabers were personalized to the individual so if you had the lightsaber of a notable Jedi it would increase value significantly to a collector.

A single non-notable functional lightsaber in-universe I would estimate at being in the many millions of credits in value. 

Anyone have any better thoughts than mine?
gpt
Possession of a lightsaber under the empire is probably a crime, or at least puts one under suspicion and garners a visit from certain agents of the empire and some sort of permanent surveillance.  For the general public this would significantly lower the potential value. For a small subset of underground collectors it would boost the price, but also make it dangerous to pursue. The Empire would not be above conducting sting operations to catch potential buyers and ensure they are not potential Force users who can't build their own for some reason.