Thanks for the replies. My wife found a good deal on Aeromexico- nonstop ATL-CUN tickets so she bought them. She's afraind prices are going to keep going up and she's probably right.
Most of the complaints about Spirit on TripAdvisor had to do with missed or cancelled connecting flights and overall customer service, or lack of it. Since we would be connecting in Miami, we would run that risk, however small that might be. I would like to try them in the future, they sure have good prices. You can't beat Aeromexico, though. They serve food and FREE drinks!
BTW: we paid about $2,600.00 for all 4 of us- no, flying ain't cheap anymore!