I cannot give you an expert opinion, but currently there are several Columbo scripts on Ebay going between about $8.00 and $20.00 -- and that is with the signatures of Peter Falk, and sometimes co-stars. One seller is asking for $70.00. I assume some are more rare than others, but to be honest I think they are something people collect more for fun and interest than as an investment. I have about a dozen, signed, including some that were unproduced episodes or "original" drafts, and I like having them (some are interesting because the character names and plot twists differ from the final episode), but would not think they are valuable as manuscripts.

Talking of scripts on eBay, do you think this is real?

https://www.ebay.com/itm/Columbo-Signed-TV-Script-X6-Murder-By-Book-Peter-Falk-Steven-Spielberg-Milner-rp/264092748495?hash=item3d7d278acf:g:VckAAOSw7XZXhdwo:rk:1:pf:0

Even if they say it's a colour copy of the script (not an original one), the photograph is clearly anachronistic for a 1971 episode, so I wonder whether the rest, autographs and all, has also been altered. And what about the F in Peter's signature? Is he known to have used that type of F in a certain period of his life? I've seldom seen it, and I don't think I've ever seen in on an early photo. A quick Google search yields this:

https://auctionimages.s3.amazonaws.com/79475/128340/120904425.jpg