You can use QueryCellFormattedText. There you would set e.Text to be what you want to see in the see. Here is a little sample code that just formats a value - but you can set e.Text to anything you want.
private void gridControl1_QueryCellFormattedText(object sender, GridCellTextEventArgs e)
{
if(e.Style.CellValueType == typeof(double) && e.Style.Text.Length > 0)
{
double val = float.NaN;
if(e.Style.CellValue is string)
Double.TryParse((string)e.Style.CellValue, NumberStyles.AllowDecimalPoint, CultureInfo.InvariantCulture, out val);
else
val = (double)e.Style.CellValue;
e.Text = val.ToString("F3", e.Style.CultureInfo);
e.Handled = true;
}
}
Here is a small sample.