According to Fred Phillips the costumes for these ambassadors were made from "Materials found stored in Paramount for as long as forty years, red and gold material dated 1939." The robe worn by the chief ambassador was one of several costumes made from bolts of rare fabric, scavenged from storerooms around the Paramount lot, for the San Francisco crowd scenes. "Originally bought for DeMille epics, often personally bought by DeMille when he thought interesting. Now rare and unusual fabrics – Paramount still has one of rarest storehouses of materials in existence, including silk gauzes, crepes and souffle for nude fill-ins as in Marlene Dietrich's nude dress (stopped weaving in France 10-12 years ago. Paramount now has possibly the largest stock anywhere)." The estimated cost for the costume was at least US$10,000, probably making it the most expensive worn by an extra. Their jewelry was made of fabric, but electro-plated. (Starlog #33, April 1980, p. 51; The Making of Star Trek: The Motion Picture, pp. 129-130)
The robe was auctioned as part of 2006's Profiles in History auction with an estimated sale price of US$1,000 to US$1,500; the final sale price was exactly US$1,000.  A later lot listed in the It's A Wrap! sale and auction contained some of the ambassador's original jewelry, and a bronze-colored robe – it sold for US$732. 
Chief Ambassador Red, black, silver and gold brocade woven with leopards and falcons, with real gold and silver wrapped around silk thread.
Assistant Ambassador Costumed in pale green-gold lame