lblFranshizShowInvwNoskhehEdit.Text = string.Format("{0:n}",
    (double)(int.Parse(drDarman["FranshizDarsad"].ToString()) *
        Convert.ToInt64(RadNumerictxtPayInvwNoskhehEdit.Text)) / 100);

string.Format("{0}", yourDouble);

string.Format("{0:0,0.########}", yourDouble);

string.Format("{0:#,0.########}", yourDouble);

string.Format("{0:n2}", double);