If you can fly weekdays you usually can get lower flights. With Spirit just make sure you understand all their baggage fees, if you pay at the gate it could be $130 for a carryon and 1 checked bag. Sun Country flies out of DFW to Cancun year round and to Cozumel in the Summer and don't forget to look at Southwest to Cancun from Love. To Cancun anything close to $200 is a good deal, $300 is probably average. Look at departing flight times, afternoon- or later is less rush to the airport and lets you dive the morning prior.