try the following solution...
Once your retrieve data from database to DataTable the add one new column in the same data table like
Assume your datatable name is dt then write,
dt.Columns.Add(new DataColumn("Total", typeof(decimal)));
foreach(DataRow dr in dt.Rows)
{
dr["Total"] = Convert.ToDecimal(dr["Salary"]) * Convert.ToDecimal(dr["Bonus"])
}
Gridview1.DataSource = dt;
GridView1.DataBind();
Now in your Grdview the add one TemplateField and add Lable in it like
<templatefield>
<itemtemplate>
<asp:lable id="lblTotal" runat="Sever" text="<%# Bind("Total") %>" xmlns:asp="#unknown" />
</itemtemplate>
</templatefield>