What is the Cheapest way to get to Kauai?

Two of my friends and I want to go to Kauai for Spring break which is the last week of March and we already have a place to stay and a car but airfare is really high. We are all students and are on a budget so we would only like to spend around $600 if at all possible round trip but the best we can find is $735. I was wondering if anyone had any suggestions on where to look. We are in Denver trying to get to Kauai. Thank you!