Use format string "0,"
Number scaling specifier: If one or more ',' characters is specified immediately to the left of the explicit or implicit decimal point, the number to be formatted is divided by 1000 each time a number scaling specifier occurs
In RichTextBox this format worked for me:
richTextBox.Text = String.Format("{0:0,}", 24000);//Show as 24
In DataGridView same format can be used in designer or by code:
//Number format "0,.###" will show number 12230 as 12.23
this.datagridview.Columns[this.dgvYuorColumn.Name].DefaultCellStyle.Format = "0,.###";