In Mormon country?! Forget it. They don't hobby; they just have multiple wives!
But seriously, prostitution is illegal in Utah, as well as all 50 states except for a few counties in Nevada.
That being said, what I've seen based on reports and some web sites, is that the agencies have amazing looking chicks, and it's not bait and switch. And the agency charges a fee of $150 or so. Sounds great, right? No. That "fee" gets you nothing. What you want beyond that is a matter of your negotiation; I doubt that it will be cheap.
But again, I wouldn't recommend doing anything in the US; just recounting what I saw from surfing.