The Spirit at Spirit Airlines
"...Only the base ticket price - Spirit has advertised base fares as low as $9 - is subject to a 7.5 percent federal excise tax. The Internal Revenue Service doesn't consider additional services essential to transporting someone so ancillary fees are not taxed.
So the fees - such as $30-$40 for a large carryon bag - are pocketed by Spirit,..."
- The Denver Post
http://www.denverpost.com/business/ci_20041506#ixzz1nVTENAdK
Some of Spitit's fees include:
$40 for a carry-on; $38 first checked bag; $45 second checked bag; $95 third checked bag. Slightly less is done on-line.
Drinks-$1-15; snack- $1-10; pets- $100
Reservations Booking Fee – $10.00 per customer, per booking
Passenger Usage Fee – Domestic Flights $8.99 to $16.99 per customer, each way
Charge for Airport Agents printing Boarding Passes (Fee starts 24 Jan 2012)$5.00 per boarding pass printed
Spirit Assigned Seating at Check-In Free
Customer-Requested Seat Assignments / Deluxe Leather $1.00 to $50.00 per seat
Big Front Seats (In advance) $12.00 to $199.00 per seat
I'd do my homework and figure out how much the flight with fees will be, then compare to other airlines.