Hello, we've been a few times as have family there.
It's not really very expensive - eating out is probably comparable to London - booze is a bit more pricey but not massively.
It's a really lovely city but do take warm clothes - we went in Nov a couple of years back and where astonished how cold it was!! Also it got dark very early.
Take refuge in cafes and eat lots of cake and hot chocolate with cream - cake is the Swedish unsung achievement IMO, the rest of their cuisine is (whisper) a bit rubbish! Hope I'm not offending any Swedish Mners...