How much do airports charge for excess baggage?

It's common to confuse two different things: the excess baggage surcharge, which the passenger pays to the airline, and airport fees, which the airline pays to the airport operator for every flight it operates.

The charge for an extra bag or extra kilos is set by each airline within its own conditions of carriage, and it's billed directly to the passenger, usually at check-in at the airport or, more cheaply, by purchasing it online in advance. The airport operator has no part in that transaction and receives none of that revenue: it's a purely commercial surcharge from the airline, not a public fee.

Airport operators — in Spain, mainly Aena — charge airlines fees for landing and takeoff, for the time the aircraft spends parked on the apron, for the use of boarding bridges and check-in counters, and a per-passenger fee that helps fund terminal upkeep and security services. These fees are calculated based on the aircraft's certified maximum takeoff weight (MTOW) and the number of passengers it carries, not on how much baggage each individual passenger brings.

Since the cost of excess baggage often appears alongside other fees and surcharges during the ticket-purchase process, it's easy to mistakenly attribute it to the airport. But except for very specific cases involving airport ground-handling of checked baggage, the airport neither sets nor collects that charge.