We did eat at a McDonalds and it was expensive. I don't remember exactly how much, but it was convenient. We also ate at the Kafe Arkadia at the Folkemuseum and I have that receipt from May 26 of this year:
Pannekakar m blabor 55.0
Toast Skinke 56.0
Sjjokoladekake 48.0
Sjjokoladekake 48.0
Kuli gronn 20.0
Totalt: 227.0
I think that was about $36 US for pancakes, a toasted ham sandwich, 2 pieces of chocolate cake and an applejuice (if memory serves - I know French and some German, but no Norwegian.) It was worth it to sit down out of the rain and have a relaxing lunch.
We were glad to have the Oslo Pass and used it that day for transportation and to see all 4 museums on the Bygdoy Peninsula, plus the resistance museum and the Nobel Museum. The only one we didn't love was the Nobel. The Castle was closed for a private event.