Using System.Globalization
...
...
...
private void button1_Click(object sender, EventArgs e)
{ 
    double valore = 123.4;
    MessageBox.Show(valore.ToString("C",CultureInfo.CurrentCulture));                                //€123.4
    MessageBox.Show(valore.ToString("C",CultureInfo.CreateSpecificCulture("en-US")));        //$123.4
}