Felleskost in this case is short for felleskostnader (shared costs). You are correct that these roughly correspond to condo fees in that they include things like daily maintenance, city fees and snow removal. But they also include your part of the payment towards the building’s shared debt. This is common for borettslag apartments. The building’s board can take loans for big projects like switching windows, redoing the roof, etc.

Since you say the felleskostnader were about as high as the estimated mortgage payments, I would guess the apartment you were looking at was a newly built borettslag apartment. New builds of that type typically have a high fellesgjeld (shared loan). The size of this shared debt is definitely something you have to take into account when buying.