I just looked at the October fares between PHL and MIA: a route I just flew on Frontier for $15, and which AA/US is clearly unhappy to have the competition.
But other than the weird (and unsustainable) $15 fares, it generally costs $30 more each way to fly AA/US instead of Frontier. That's pretty much Frontier's bag fee. So unless you're the unusual person who can travel with just a backback or insist on checking a bag instead of carrying it aboard, it's hard to understand why you'd fly Frontier unless you were unfamiliar with their additional fees.