I'm not from Iceland but I was there last March for 5 days. Fuel is about the same price as the UK, quite a few of the stations are self service i.e. you put your card in the pump to pay, there is no kiosk to pay at. Food is a little more expensive especially in the international chain type places. There are some nice local restaurants in downtown Reykjavik. If you're hiring a car then I'd highly recommend getting something with 4wd, some of the places we went to we would have got well and truly stuck otherwise. Just going round the Golden Circle would have been difficult with 2wd.
Use this site
http://www.road.is/ to see what roads are open when planning a trip as even the major roads can temporarily close in winter (Route 1 if you're going east I would imagine), absolutely stunning scenery on that road especially in the highland part.
All of the things we did were pre-booked so we only needed cash for food/fuel/souvenirs etc I think we took around 60,000ISK each which is around £300.
It's an awesome place to go to, definitely going to go again someday.