From my experience the approximate living costs presented at KI website are pretty accurate. However, I have some comments on this sensible and important aspect of being a student in Stockholm.
Sweden indeed is a very expensive country. (I guess it is the price one has to pay for having a developed economy.) A way how you can cut down on food is to cook yourself and take lunchboxes to school. So start looking up some recipes and remember that the freezer is your friend!
Today the accommodation costs are higher than KI has presented (I am again referring to this KI page.) I was lucky enough to find places to rent for 3000-4000 SEK, those being rooms in family apartments. However, you should be prepared to pay around 4500 SEK per month for a room. (For my experience with accommodation read my blog post “Accommodation Hunting in Stockholm”.)
Insurance: As I am an EU citizen with the EU insurance card, I don’t have to pay anything for it.
Phone: I’m using a pre-paid Comviq SIM-card. For 100 SEK I can talk for 200 min within Sweden, send numberless text messages and surf I GB a month (Fastpris Mini).
Leisure: A dinner in most places will cost you 150 SEK. For a beer in a nice bar you’ll spend at least 60 SEK. That is one reason why the MF Friday pub at KI is awesome! A beer/cider costs only 30 SEK ^^
Stockholm is a wonderful, yet expensive city to live in. Thanks for reading!