how to convert/cast this to a double

I'm trying to convert or cast this string value of the texbox, txtPrice, to a double and assign it to the default value of the
update parameters.  I can't seem to get it right though.
How should the casting be done for this?
ProcedureDetailsDataSource.UpdateParameters["Price"].DefaultValue = ((TextBox)ProcedureDetailsView.FindControl("txtPrice")).Text;


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