How are you declaring your datafield in the datasource? What the error is relating to is that the datasource is trying to map the fieldname from the datarecord to that gridview column, but the column is already set as a calculated field.
To get around this, adjust your select query to only contain the fields that match columns; excluding fields that coorespond to a calculated field.
For example:
Select Price, Quantity from Database = datasource to Gridview that contains the following columns:
Price, Quantity and your calculated column Total.