Frequent travellers of Arlanda Airport, as well as airport staff, all know the cheapest beer at Arlanda can be found at McDonald's in Skycity.
One large beer at McDonald's is 65:- (6€).
McDonald's and Skycity can be found in the departures area, before security, between terminal 4 and 5, it's a quick walk from all terminals. If you're in terminal 2 you can take the Arlanda Express for free between the south and north station.
Why is this special? It's the only McDonald's in Sweden with permission to sell alcohol.
Keep in mind that it can take 25 minutes to pass the security and get to the gate and that the gates usually close 30 minutes before the departure time, so leave at least 1 hour before departure.
Because of the way Arlanda Airport is being run (all stores and restaurants are run by the same company (as franchises), just different brands) there's no actual competition at Arlanda and prices will never drop. McDonald's has had the cheapest beer (and food) for many years.