Kevin,
my most sincere apologies for not replying and thanking you sooner. Your
code was a great help and with a very minor change I was able to get exactly
what I wanted. My C# code I ended up using was as follows:

protected void LoadData()
{
     //some code to load dataset, add primary key to ID column,
     //add datarelation between ID and ParentID columns

     ds.Tables["MyTable"].Columns.Add("cumulative",
typeof(System.Decimal));

     DataView dv = new DataView(ds.Tables["MyTable"]);

      //Set the row filter to be root nodes in the tree
     dv.RowFilter = "ParentID IS NULL";
     dv.Sort = "ID asc";

     foreach(DataRowView dr in dv)
    {
        dr["cumulative"] =  Summary(dr.Row);
     }

     this.ultraGrid1.DataSource = dv;
     this.ultraGrid1.Refresh();

}

protected decimal Summary(DataRow dr)
{

  decimal ret = 0;
  ret = (dr["cost"] == System.DBNull.Value) ? 0 :
Convert.ToDecimal(dr["cost"].ToString());
  DataRow[] childRows = dr.GetChildRows("myRelationName");

  decimal childTotal = 0;

  foreach(DataRow child in childRows)
  {
     childTotal = Summary(child);
     child["cumulative"] = childTotal;
     ret += childTotal;
  }

  return ret;
}

the dataview was bound to an Infragistics UltraGrid which allowed me to
display the data in a tree-like datagrid. Thank you again for your
help....it was most helpful!!!

Regards

James