The cost of renting a luxury car can vary depending on several factors such as location, rental duration, demand, and availability. However, some of the cheapest luxury cars to rent may include models like the Audi A3, BMW 3 Series, Mercedes Benz C-Class, or the Lexus ES. It's important to note that prices can differ between rental companies and destinations, so it's recommended to compare prices from different rental providers to find the best deal.