it is such that I must have written price in a string to be displayed to users.
right now will give this error is
Operator \'/\' cannot be
As far as I can tell you, prisenMedlemskab is a string and not an integer You have to parse it.
prisenMedlemskab
e.g.
double prisenMedlemskabDouble = double.Parse(prisenMedlemskab);
And then do
ButtonPay.Text = "Buy Membership - DKK " + (prisenMedlemskabDouble / moms).ToString("F2") + ",- Kr";