Remember that Betfair takes 5% of your winnings, so with the same odds to back at Betsson and lay at Betfair, you have to lay slightly less on Betfair and bet slightly more on Betsson.
When you "Dutch" (so that you win or lose the same, regardless of the outcome) a back and lay bet with the same odds at Betfair, your expected loss is 2.5%, or 37 kronor, 50 öre.
Here's a site
http://www.ifreebet.co.uk/betcalculator
that will find you a football match and has a calculator for your back and lay bets that will tell you your expected loss (or win if it's an arb).