Piggybacking on what sdsearch and BearX220 said.

Frontier often has great deals ($15 one-way) especially out of cities like Atlanta, Austin, Chicago, etc. You DO have to pay for your luggage (~$20 to check a bag and ~$25 for a carry-on---on way), so unless you're traveling extremely light, you're going to automatically tack on $40-50 on whatever the quoted fare is. You also have to pay for your seat, but I recently learned that you can purchase your ticket without selecting your seat and save a few bucks.

It's still a great deal depending on the scenario. Say you're a couple and you only take one checked bag. Or this past May, I flew RT from Austin to Atlanta for about $60RT, including the cost of my seat and carry-on (which is pricier than checking).

Just don't expect free snacks and all kinds of perks. It's on-par with Spirit-type airlines.