It seems to be everyone's first question about Norway. And yes, it is indeed expensive.
I won't tell you how much we spent in five days there, but if you count on spending a third more than you would in the UK you won't be far wrong. Usually I use cash for nearly everything, but here I switched to using my Transferwise debit card, onto which I had put more Norwegian krone than we could possibly spend (though we came closer than expected), and tried not to think about prices.
We did quite a bit of travelling: the express train from the airport into Oslo, trains from Oslo to Myrdal and then down to Flåm, a taxi to Aurland, the bus tour up to the Stegastein viewpoint, the ferry from Aurland to Bergen, and two taxi trips in Bergen. And there's not much accommodation in some areas, so our hotel in Aurland was expensive. We also ate out quite a bit, which is around 50% more expensive than in the UK; food bought in the supermarkets was similarly expensive. Those were the (relatively) most expensive things.
Other things seemed relatively cheap, or at least only normally expensive.
Our hotel in Oslo, the Thon Spectrum, was (on special) 4800 kr (about £450) for three nights, which seemed roughly on a par with UK rates for similar hotels. It was a nice hotel, and had probably the best hotel breakfast we've ever found.
The museums we visited weren't expensive: 130 kr per adult for the Norske Folkemuseum, 100 for the Viking Ship Museum, and 120 for the National Gallery. And children were free at all three. (For comparison, the Folkemuseum is only two thirds as much as the Weald and Downland Museum near Chichester.)
I bought a guide to the buildings at the Folkemuseum. This is a really nicely produced 160 page hardcover with full colour photographs, including a general introduction as well as descriptions of the buildings and their background, and I thought I'd misheard -- possibly missing a digit -- when the cashier told me it was only 99 kr. (The other book we bought was Egner's When the Robbers Came to Cardamon Town, recommended in a bookshop as a Norwegian classic; that was 250 kr, but was a nice hardcover edition. And Helen almost convinced me to buy a biography of Edvard Grieg at the airport on the way home, after I read her the first few pages; that was 200 kr, an ok price for a small hardcover.)
Public transport in Oslo is prima facie more expensive than London or Oxford, with a basic ticket 35 kr (about £3) or 18 for a child. But this gets one access for an hour to all transport in Zone 1, which at about 20km E-W and 30km N-S includes pretty much the entire city (and 105 kr gets one a day ticket): so actually cheaper for many purposes, and certainly simpler and better integrated. Oslo public transport is also clean, comfortable, high frequency, and comprehensive. The express train from the airport into Oslo central (47km) was 190 kr per adult, which seems comparable to the Heathrow Express, though the pricing for the latter is much more variable. In general public transport pricing in Norway seemed less likely to shaft occasional or out-of-town users than in the UK.
For more comprehensive numbers, see this cost of living comparison. We should clearly have tried to find some way of putting Helen into daycare for a week!