Location factors into rental car prices in two ways: the city you rent in and where exactly you pick up the car (e.g., at the airport).
Let's start with where in the world you will be renting. Rental car prices vary a lot from country to country and city to city. You may be surprised to learn that the prices in many of the most popular tourist destinations can actually be significantly lower than in other places. This is because the huge demand drives many companies to enter the market and, therefore, lots of competition. For example, rental cars in
Lisbon, our most popular location, can be had for as little as €2 per day.
Other locations that aren't as popular may have only a few companies serving them and, therefore, higher prices. Finally, some locations that are harder to get cars to, like islands, also have significantly higher prices.
Now let's focus on where you pick up and drop off your rental car. This, surprisingly, can make a big difference in how much you pay. Most often, you will pay more to pick up your rental car at the airport than at a location near the airport.
Rental car companies pay a lot to have rental counters in airport terminals or in the parking lot of the airport. They then pass these costs on to customers either through an airport fee or indirectly in the rental rate. It is, of course, more convenient to pick up your car at the airport, though.
But if you are willing to take a little more time and take a shuttle from the airport to the rental car company's location nearby, you will likely be able to save a significant amount.
Occasionally, it may be cheaper to rent a car in the center of a city rather than at the airport. This is usually not true at larger airports that have a lot of rental companies located near them (not necessarily at it). Usually, only the largest companies have rental offices in cities and these tend to be more expensive.
At smaller airports, though, it may be cheaper to take a taxi or public transportation into the city and then pick up a car there to save on paying the airport fee. Keep in mind though that downtown locations are usually only open during normal business hours; this could be inconvenient if you have an early or late flight.