Choosing a seat on a flight increasingly carries a charge, and the amount varies sharply by position. The pricing tracks preference rather than any difference in what the airline provides.
The seat costs the airline nothing extra
Every seat on the aircraft flies whether or not it is chosen in advance, and the fuel, crew and handling cost is identical for all of them.
The charge is therefore not recovering a cost. It is pricing the option to avoid an unfavourable outcome from random allocation.
That makes seat selection a near-pure margin product, which is why it has expanded so quickly across the industry.
Preference maps directly onto price
Extra legroom rows carry the highest charge, followed by rows near the front where passengers disembark sooner, then window and aisle positions.
Middle seats towards the rear are usually free or cheapest, because nobody chooses them voluntarily.
The resulting price map is essentially a survey of passenger preference expressed in currency, refined continuously by which seats sell.
Exit rows behave slightly differently, since safety rules restrict who may occupy them, and the airline has to weigh the fee against the chance of reseating someone at the gate.
Group travel strengthens the incentive
Automatic allocation frequently separates passengers on the same booking, particularly on full flights.
Families and groups therefore face a choice between paying to sit together and risking separation, which converts a preference into something closer to a necessity.
Some jurisdictions have examined whether allocation systems separate groups more often than chance would, and rules on this vary by country and continue to change.
Fare bundles reprice the same seat
Airlines sell fare types that include seat selection, baggage and changes, alongside a basic fare that includes none of them.
The bundle is priced so that buying the components separately costs more, which pushes passengers up a tier.
Comparing the total for what you actually need, rather than the headline fare, is the only way these bundles can be compared honestly across airlines.
Waiting until check-in changes the odds
Unsold selectable seats become available free at check-in, since the airline gains nothing by leaving them empty when allocation happens anyway.
On lightly loaded flights this reliably produces a decent seat at no cost, and on full flights it produces whatever remains.
The trade is between certainty and cost, and it depends on how full the flight is, which is information the booking page does not provide.
Seat maps shown during booking give a rough indication, though they exclude seats held back for elite passengers and crew, so an apparently empty cabin can fill later.