The value of screenplays can be tricky to determine, but don't be mislead by false advertising, such as sellers proclaiming "signed" scripts. The only real value a screenplay has is its rarity (it is difficulty to find the screenplay because its never been published, etc), or its "provenance" - in other words, it's history. Anyone can get a copy of a screenplay and run to the copy center, and -- if they get fools to put up $$$ -- roll in the dough. But what screenplays are really valuable?

First, a lesson in the process of filmmaking. Screenplays are written, then registered with the Writers Guild (WGA). Next, the writer 's representative (agent, manager, etc) pitches the screenplay to the money people -- i.e., the Studio. Once  money has been put on the table, the screenplay goes through revisions before production. Lots of writers can be involved in the revisions. If the financing approves of all the changes, then the script goes into production. During production, as changes are made, the revised scenes are replaced by colored pages so that it is easy to determine whether you are looking at the most recent draft of the scene's dialog. The Script Supervisor is in charge of noting the number of takes and other changes, including the director's take preference. The Director has their personal script on which they write directorial notes. Once the shooting is done, the script can change during editing. Post editing, there is a script which is produced for marketing, i.e., for foreign subtitling.

So how do you determine the value of a script? If you love a film and want a copy of a screenplay which is not available anyplace else (it's not available in print), you'd have to make the decision how much you want to spend and leave it at that. BUT A SCRIPTS TRUE VALUE WOULD BE DETERMINED BY WHERE IT FELL IN THE CHAIN OF PRODUCTION. The  writer's original draft (we are talking pre-computer printouts here) and the director's working script having the most value. Actor's script are, of course, valuable, especially with the actor's notes. If you had all versions of a film's script, the value of your script would increase.

We are talking about REAL VINTAGE SCRIPTS, not  xeroxed copies of a script which someone is making at the copy center.

And SIGNED SCRIPTS?  You can do what a lot of the unscrupulous script sellers do, which is get a COPY OF AN AUTOGRAPH and do a mock up page with colored signatures.  Forget about value. You could say they're making money because people are buying them. It's hard to authenticate signatures, but if you have a copy of a signature, it's worthless. You'd be better off with a Vintage Screenplay.

And please, if you do buy a screenplay, don't make copies.  Its one thing if a screenplay was copied and handed out by the writer's agent, quite another if you are making copies and pirating it on ebay.