private void textBox1_TextChanged(object sender, EventArgs e)
{
    if (textBox1.Text == "" || textBox1.Text == "0") return;
    decimal price;
    price = decimal.Parse(textBox1.Text, System.Globalization.NumberStyles.Currency);
    textBox1.Text = price.ToString("#,#");
    textBox1.SelectionStart = textBox1.Text.Length;
}

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