The same car from the same company frequently costs less from a city depot than from the airport terminal a short distance away. Several distinct costs sit behind the difference.
Airport concessions are charged as a share of revenue
Rental companies operating at an airport pay for the privilege, usually as a percentage of the revenue generated there rather than as a fixed rent.
That charge scales directly with the rate, so it is recovered by raising the rate rather than absorbed as a fixed overhead.
It frequently appears on the invoice as a separately named fee, which is why airport rentals carry line items that city rentals do not.
Airport operations cost more to run
Desks staffed across the full span of arrivals, shuttle buses to remote lots and parking within the airport perimeter are all substantially more expensive than a city forecourt.
Fleets held at airports must also be larger relative to demand, because arrival peaks are sharp and a customer who arrives to no car is a serious failure.
Those idle vehicles carry depreciation and financing costs whether they are rented or not, and the rate has to cover them.
Demand at the terminal is captive
A traveller who has just landed has limited alternatives, little inclination to compare and luggage to manage, which weakens their position considerably.
A customer in a city has other depots within walking distance and can decline a price, which forces those locations to compete more directly.
Pricing systems know the difference, and airport rates are set with the knowledge that most bookings will not be reconsidered on arrival.
One-way and cross-border rules differ by location
Airport depots handle far more one-way rentals, and repositioning vehicles back to their base is a real cost recovered through drop-off charges.
City depots often restrict one-way returns entirely, which is one reason their rates can be lower but their flexibility is not comparable.
Cross-border permissions and the associated fees are also set by location and by country, and the terms vary between operators.
When the airport is still the better choice
The saving disappears if reaching a city depot requires a taxi, and it disappears entirely for a party with substantial luggage.
Opening hours matter as well, since city depots close in the evening and at weekends while airport desks track flight schedules.
Collecting in the city at the end of a stay rather than on arrival often captures the saving without the transfer cost, because the journey is already being made.