This issue occurs because the generated Excel report Price column width is not wide enough to support the data format. A workaround for this issue would be to add some whitespace to the Price column caption during Export server side event so the column width in the Excel will be wider. Here is the snippet:

protected void wgTest_Export(object sender, ISNet.WebUI.WebGrid.ExportEventArgs e)
{
    e.Table.Columns.GetNamedItem("Price").Caption = "Price     ";
}

Add whitespace to column caption does address the issue.  I also tried to set column width during Export event but that seems doesn't help. This sounds like a bug of WebGrid export function.

Thanks for your help Glenn!