The value of the 1st place prize is $4200, not $1500:
http://www.filmracing.com/100/prizes.htm
I'm not convinced that the prize is what matters. The winners of the competition will receive a lot of exposure, which is the actual end game ... not the prize value.
Regardless, Shannon auditioned because she wanted to participate and she saw value in it. She was chosen for the part during the audition. She was going to be working on that production from Dec 4-8. It's unrealistic to think that she took the time to audition in Edmonton for the part, but she didn't want the part, or that she was actively pursuing a successful career, and then decided throw it away by vanishing.
I'm convinced that Shannon is missing against her will, and I'm not optimistic that this will have a good ending.
ETA: Check out some of the winning films. The competition is clearly an international event.