There are many hire car companies, and the big internationally recognised names are often the most expensive! Here are a few things that I do to find cheap hire cars:
Use Skyscanner to look for your Hire Cars. I prefer Skyscanner as it searches across more of the smaller travel agencies and doesn't get sponsored by the big hire car companies (Avis, Hertz, Europcar) to promote their vehicles above cheaper alternatives.
When you find a hire you're interested in, it will show you all the possible options for which travel agency you can rent the car with. See below, we've searched for a car in Reykjavik from November 4th to 11th, and below are three of the options that came up:
I love Dacia Dusters for sleeping in - they're a fuel efficient car and the back seats lie flat with a short incline. If I click on this, it will then show you options for third party "OTA"s (Online Travel Agencies) that you can make your reservation through. If the price is signficantly cheaper through an OTA, you can make the reservation directly with them. Do make sure to check the reviews of any of these agencies using TrustPilot before booking though.
Personally, I prefer to book direct through the agent that owns the vehicle, especially if it's a smaller rental company, as this means all the money goes directly to the owner, rather than the OTAs taking a commission. I still use SkyScanner to find the rental car, but now I'll show you how to get in contact directly with that company.
Following the guide above and using Skyscanner, you'll be able to narrow down your options to the cheapest hire car companies. In the above example, the cheapest option for a 7-day hire of a Dacia Duster in Iceland is showing as €243 through "Enjoy". Click on "Select" (in the red box) and it will take you the "Enjoy" booking platform where you can reserve through the OTA.
In the below screenshot, there are two red squares. The one at the top left shows you that we've left Skyscanner, and we're now on the "Enjoy" travel booking platform. However, the second red square (centre bottom of the page) shows the "FairCar" logo. This is the name of the company that owns this Dacia Duster, and this is the actual company that you'll be hiring your car from. If you click on that FairCar logo, it will now tell you more information about it, including the vehicle location (the hire car company's address - see red square in the right photo).
Please note, when you're booking through third party companies, it will often say things like "24 Hour Assistance" and "Free Additional Driver" - read the fine print on these! 24 Hour Assistance may be available, but that usually just means you can speak to a call centre in the Phillippines at any hour of the day or night, and they won't be able to provide immediate support. In the below screenshot, the "Free Additional Driver" is also inaccurate - when you click on the next page you'll see that is €55 extra. If you're going through an OTA, the OTA doesn't usual provide you with any insurance - so it will depend on what insurance the rental car company actually provides. OTAs are notorious for false advertising of these things.
Now that we know the rental company and their address, google "FairCar Iceland". You can check it's the correct place by checking the address. This has now given us the website and phone number of the hire car agency.
On the FairCar website, we can see the insurance that is actually provided - just basic cover. Theft protection and collission damage waiver is an extra cost, not included. FairCar is advertising the car for €279, instead of €240 as advertised on "Enjoy".
The next/last thing I would do it send a polite email to FairCar asking if they'll price match. Send them the screenshot for the Enjoy booking, and ask them if they can match the price to €240 for you to book direct. Email addresses are usually on the bottom of the page or under a "Contact" form. The Hire Car company has no obligation to say yes, but if you're polite they usually do.