Yeah according to this Iceland has the 3rd highest gas prices in the world:
https://www.globalpetrolprices.com/gasoline_prices/
But I actually just got my CC statement and tallied up all my gas purchases and it worked out to be around $460 for 10 days so not AS bad as I had feared. However I also received a notice from the car rental company saying I got a parking ticket so they're now billing me the parking fee $7.50 + $25 Admin fee. LIke everything in Iceland parking is 100% automated so I missed the pay station/signs and was caught by the camera on the way out. FML
The airline stopover in Iceland is like how they tempt you into buying a printer. Super cheap upfront cost but they get you on the variable costs.
But yeah despite all that it's a totally unique place and would recommend anyone to visit before it gets (even more) overrun.