Excellent prices.... a few examples:
Kingly (or queenly?) first-rate meal of fresh fish etc., ~$20-30. Even rather pricey locations were serving dinner at roughly that price point. Notwithstanding the price of alcohol in Scandinavia in general, good cuisine was very affordable.
Rental car, stationwagon, $50/day; some models for ~$35/day were available.
Family "cabin" in Hveragerði (good location proximal to many of the better-known sights) with lounge, kitchen, hot-tub, and sleeping space for 4: ~$80/day
Comfy 3-person room at the very nice Northern light Inn (beside the blue lagoon) with shower etc: ~$100/day. We stayed there on our first and last nights, as it is such a nice treat to be able to walk over to the lagoon for a dip and also be within a very short drive from the airport.
Petrol prices were way lower than I ever recall them being in Iceland. I don't remember the number but it was the first time I've been in Iceland and not felt completely robbed every time I filled up.
Overall, I'd put the cost of living/vacationing at roughly the same as where I live in the US. So, quite economical, actually.