private void setCustomSummary() {
   treeList1.OptionsView.ShowRowFooterSummary = true;
   treeList1.Columns["Budget"].RowFooterSummary = SummaryItemType.Custom;
   treeList1.Columns["Budget"].RowFooterSummaryStrFormat = "{0} nodes exceed limit";
}

private void treeList1_GetCustomSummaryValue(object sender, DevExpress.XtraTreeList.GetCustomSummaryValueEventArgs e) {
   if(e.Column.FieldName == "Budget") {
      IEnumerator en = e.Nodes.GetEnumerator();
      int exceedingLimitNodes = 0;
      while(en.MoveNext()) {
         TreeListNode node = (TreeListNode)en.Current;
         decimal budget = (decimal)node.GetValue(e.Column);
         if(budget > 500000) {
            exceedingLimitNodes ++;
            //...
         }
      }
      e.CustomValue = exceedingLimitNodes;
   }
}

Private Sub setCustomSummary()
   TreeList1.OptionsView.ShowRowFooterSummary = True
   TreeList1.Columns("Budget").RowFooterSummary = SummaryItemType.Custom
   TreeList1.Columns("Budget").RowFooterSummaryStrFormat = "{0} nodes exceed limit"
End Sub

Private Sub TreeList1_GetCustomSummaryValue(ByVal sender As Object, ByVal e As DevExpress.XtraTreeList.GetCustomSummaryValueEventArgs) _
Handles TreeList1.GetCustomSummaryValue
   If (e.Column.FieldName = "Budget") Then
      Dim en As IEnumerator = e.Nodes.GetEnumerator()
      Dim exceedingLimitNodes As Integer = 0
      Dim node As TreeListNode
      While en.MoveNext() = True
         node = CType(en.Current, TreeListNode)
         Dim budget As Decimal = CType(node.GetValue(e.Column), Decimal)
         If (budget > 500000) Then
            exceedingLimitNodes += 1
            '...
         End If
      End While
      e.CustomValue = exceedingLimitNodes
   End If
End Sub