There is no way to track the market, except to watch for trends and the fluctuations, which are all about supply and demand. Many times auctions can have a greatly inflated price because of two collectors in a bidding war. People do get crazy at auctions.
Check closed auctions on sites that deal with documented props and watch the trends on fixed price web sites.
Sold Prices for Assorted Movie Props
Note: Image is an "Andrew" robot suit worn by Robin Williams in the movie Bicentennial Man valued at approximately $15,000. The piece is from ScreenUsed.com's personal collection.