The math has been done already, widely.  I mean, what do you think all of those analysts on wall street do with their spreadsheets?   Anyway, the metrics that matter in the airline biz are Revenue per Available Seat Mile, and Cost per Seat Mile ( RASM and CASM).   The difference is the gross margin per seat mile.  
Feel free to look at the numbers yourself, but I can tell you - Spirit has one of the highest margins of any carrier.  Their costs are about the same as Frontier or Allegiant ($0.085 / mile or so), while their revenue is among the highest. If we assume fuel costs are similar everywhere, most of Spirit's profit comes from having lower than average labor costs.
American and Delta, et al, have higher cost structures, but also have far better revenue.  Delta probably gets about $0.15/ seat mile, with costs around $0.12.