Use the DisplayFormat property on the SummarySettings object.

        private void ultraGrid1_InitializeLayout(object sender, Infragistics.Win.UltraWinGrid.InitializeLayoutEventArgs e)
        {
            UltraGridLayout layout = e.Layout;
            UltraGridBand rootBand = layout.Bands[0];
           
            SummarySettings ss = rootBand.Summaries.Add( SummaryType.Sum, rootBand.Columns["Decimal 1"]);
            ss.DisplayFormat = "{0:c}";

        }