Routinely see Delta and United for ~$125 round trip on that route. You do have to pay extra for carryon and/or checked bags on Frontier, so be careful there. I've found Southwest is no longer the low cost leader it once was, and prices are often cheaper on the Legacy carriers. With a Delta hub in SLC and a United hub in DEN, plus heavy SWA and Frontier presence, you can usually find cheap fares on this route - don't just limit yourself to SWA.