You have to implement the ICustomSummaryCalculator interface and assing the implementation to the SummarySettings.CustomSummaryCalculator property.

Example:
SummarySettings ss = this.ultraGrid1.DisplayLayout.Bands[0].Summaries.Add(SummaryType.Sum, this.ultraGrid1.DisplayLayout.Bands[0].Columns["Summary"]);
ss.SummaryType = SummaryType.Custom;
ss.CustomSummaryCalculator = new SummaryCalculator();

public class SummaryCalculator : ICustomSummaryCalculator
{
    private double total = 0;

    public void AggregateCustomSummary(SummarySettings settings, UltraGridRow row)
    {
        string cellValue = row.Cells[settings.SourceColumn].Text;
        double val = 0;
        if ( double.TryParse(cellValue, out val) )
            this.total += (val / 1000);
    }

    public void BeginCustomSummary(SummarySettings settings, RowsCollection rows)
    {
        this.total = 0;
    }

    public object EndCustomSummary(SummarySettings settings, RowsCollection rows)
    {
        return this.total;
    }

}