This is being auctioned by Kim Metzger. His notes are below, and they really surprised me about his connection to the costume! I recall the changeover to that costume in the early to mid 70's, and it was one of my all-time favorites also. I don't recall reading anything about who designed the costume, but maybe there was a design contest back then sort of like they had for Supergirl.

Steve Z (eBay Boy)

From Kim: This is a beautiful 11x14 pencil sketch of Saturn Girl of the LEGION OF SUPER HEROES, drawn by Mike Grell, who got his start with the Legion.
In this picture, Saturn Girl is in the cutaway outfit she wore throughout the 1970s, a costume I'm very fond of, especially since I'm the one who DESIGNED it!

<img src="http://legionpics.net/images/LP00079 - saturn girl Grell.jpg" alt=" - " />