As far as I can tell you, prisenMedlemskab is a string and not an integer
You have to parse it.
e.g.
double prisenMedlemskabDouble = double.Parse(prisenMedlemskab);
And then do
ButtonPay.Text = "Buy Membership - DKK " + (prisenMedlemskabDouble / moms).ToString("F2") + ",- Kr";