Web Forms :: Assign A Currency Formatted Textbox Value Into A Double?

I am formatting textbox currency values as follows:

this.MyTextbox.text = String.Format("{0:c}", dblTotalNrcIrapContributionAmount);

I need to copy the values from a textboxes text property into a nullable double, currently I have:

//test for empty string - if yes set dblMyAmount to null,
// else set value to this.MyTextbox.Text value
Nullable <double> dblMyAmount = ((this.MyTextbox.Text +
"").Trim() ==
"") ? (Nullable<double>)null :
Convert.ToDouble(this.MyTextbox.Text);

which is not working - the error message is "Input string was not in a correct format."