FLL to Dallas jumped from $39 to $124 within 48 hours of Spirit’s May 2 wind-down. Two weeks in, the routes Frontier didn’t backfill are still up 20%.

24/7 Wall St ran the numbers and pegged the peak at 218% on Spirit’s busiest trunk routes within 48 hours of the wind-down. Their cleanest example was Fort Lauderdale to Dallas: a $39 Spirit fare on Friday, May 1, hit $124 by Sunday night, May 3, on the legacy carriers stepping into the gap. The math is a 218% increase on the base. The replacement ticket is 3.2x what Spirit charged.
Marketplace’s reporting the same week put the sustained estimate at 15% to 20%, citing a spokesperson from Going.com. The wider Cirium and Kayak real-time data ran 15% to 25% across every route where Spirit had double-digit market share. The 218% number is the peak. The 15% to 20% number is what is sticking.
A great solo travel tip spotted this week on Live and Let's Fly.