formatting DataColumn.Expression

[J]

Is there anyway to format a calculated datacolumn? I would like the
following column to display as money (formatted as $#,##0.00). Or how about
simply displaying the column formatted as a number with only 2 decimals
(#,##0.00)?
In the sample below, amount and taxrate are existing DataColumn's.

DataColumn colTax = new DataColumn();
colTax.DataType = System.Type.Get Type("System.St ring");
colTax.ColumnNa me = "tax";
myTable.Columns .Add(colTax);
colTax.Expressi on = "String.Format( \"{0:C}\",\"amo unt*taxrate\" )"; //error
: Expression does not support String.Format