Not just renting, but everything in general is expensive. Truly, on the order of Japan expensive. Be prepared to spend a fair chunk of change, especially if driving a 4x4, and if you are coming from the U.S. with our ever weakening dollar. Plus, 4-wheel drive vehicles in Iceland are serious machines. There are tons of retired folks camping out of Unimogs which will make your vehicle feel like a VW Rabbit. And, to reiterate the previous advice, when a road is said to be "rough" or "crosses a stream" - it is generally an understatement. But, Iceland is a huge playground, so have fun.