We flew Allegiant for the first time our last trip and had a good experience.
There were a lot of fees. Baggage being the main one. They charge for carryon bags. Also their overweight fee for checked bags starts at 40 lbs instead of 50. For us it was cheaper to pay for 1 checked bag and not bring carryons.
There were also fees for priority boarding and seat assignments. We didn't purchase any of this, but were able to sit together both flights.
The other cost are tolls if you are renting a car. I'm guessing we spent an extra $20 to $30 on tolls.