Costello, who competes at the University of Utah, shot a 6-under 210 at the Purple Sage Golf Club in Evanston.
University of Wyoming sophomore-to-be Caitlyn Skavdahl (Natrona County) finished second with a 4-over 220 to finish one stroke back of Ali Mulhaal.
The Cowboys had three golfers place in the Top 10 -- Laramie's John Murdock was third with a 216, Dan Starzinski was fifth with a 218 and Cheyenne's Jared Edeen was ninth with a 222.