The attached example works incorrect in my project (in 9.3.2), corrected part below:

 protected void ASPxGridView1_CustomSummaryCalculate(object sender, DevExpress.Data.CustomSummaryEventArgs e) {
        if(e.IsTotalSummary) {
            if(e.SummaryProcess == DevExpress.Data.CustomSummaryProcess.Start)
                e.TotalValue = 0;
            if (e.SummaryProcess == DevExpress.Data.CustomSummaryProcess.Calculate)
            {
                if ((sender as ASPxGridView).Selection.IsRowSelected(e.RowHandle))
                    e.TotalValue = Convert.ToInt32(e.TotalValue) + Convert.ToInt32(e.FieldValue);
            }
        }
    }