I have 3 costumes at home that i bought for 100 dollars. One of them was worn by the 'betrayer' tokra chick in Tok'ra, i've never been able to ID the others. Extras costumes are rarely labeled what show they're for, they're used, cleaned, tossed in a bin or hung up and that's it.
after 10+ years of a show, you can imagine that they'll have tons and tons of extras costumes, most of which are hand made.
Memorbilia can be very cheap, it just depends on how much a part of the show it was.
Like they had on an episode of Pawn Stars, when a guy thought he had a real whip from Indiana Jones
roughly the expert said : if this was a prop one of the stunt guys brought on set to play with, eh, few bucks to few hundred. If it was used in the movie by a stunt man, several hundred to a couple thousand, if you can prove that Harrison Ford used it in the movie, complete with certificate of authenticity, 30-40,000.
same whip, the pedigree determines its value
same with show props. the more recognizable and memorable, the more sought after it'll be. Background stuff that they changed out every so often or that there were a dozen copies of, very affordable.
Hero costumes are often some of the most sought after items. I've been at con auctions and seen them go for hundreds or thousands. I saw the Crystal Skull sell for 2400 several years ago.
and then i have my two props that were sold at silent auction and i was the only bidder.