Caitlyn Jenner's Used Outfit For Sale ... And It Ain't Cheap!!

The Caitlyn Jenner collectible market is BOOMING ... at least one auction house is hoping it is ... selling one of CJ's used racing suits -- and they're hoping to score $20,000 for it!!!

The item is the driving suit Caitlyn (then Bruce) wore during the 1982 Celebrity Toyota Grand Prix of Long Beach ... a race Bruce dominated and won!!

The suit eventually came into the possession of a collector who has decided to sell now that Bruce has become Caitlyn and Caitlyn has become a cultural icon for obvious reasons.

A rep for Nate D. Sanders, the company running the auction, tells TMZ Sports the starting bid is $5,000 ... but they're expecting bids to reach as high as $20k.

Hope you saved up!