The same 15-pound dog in the same under-seat carrier costs $74 round-trip on one airline and $300 on another.
Most pet owners book flights by schedule and price, then discover the pet fee at checkout. The fee varies 4x depending on the carrier, and nothing about the service changes between the cheapest and most expensive.
WestJet charges $37 one-way. That is $74 round-trip for the exact same under-seat pet carrier every airline requires. Allegiant is $50 one-way ($100 round-trip). Both are lower than Delta's $95 one-way, which most travelers assume is standard.
The middle tier clusters around $100-$125. Delta charges $95, Alaska $100, Southwest $125, JetBlue $125. Spirit also charges $125, which means Spirit's pet fee matches Southwest's despite Spirit being the "budget" option. I keep per-airline policy pages at carry-on-checker if you want to check pet fees, bag limits, and special item costs for a specific carrier.
American and United both charge $150 one-way. That is $300 round-trip. For two pets on a family trip, that is $600 in pet fees alone on a single domestic round-trip.
Nine airlines charge under $100 one-way. WestJet, Allegiant, Eurowings, Gol, Breeze, Sun Country, Air Canada, Delta, and Frontier. The cheapest five are all under $75.
The airline with the lowest base fare is not the cheapest airline to fly with a pet.
