The FormatCurrency function is used in VBScript. It's only in VB.NET for backward compatibility with older code. You're also passing in a String value, not a formattable number.
Convert the text in your
txtprice
TextBox to a numeric type, like Single, Double, or Decimal and you can then pass it to String.Format:
Dim price As Single = Single.Parse(txtprice.Text)
txtPrice.Text = String.Format("{0:C}", price)
This is just a quick example and nowhere near what I would actually do in production quality code.