<asp:TemplateColumn>
        <HeaderTemplate>
            HeaderTemplate
        </HeaderTemplate>
        <ItemTemplate>
            <%# Eval("ColonnaTabellaDB") %>
        </ItemTemplate>
        <FooterTemplate>
            <asp:TextBox runat="server" ID="Tb1"></asp:TextBox>
        </FooterTemplate>
    </asp:TemplateColumn>

protected void Grid_ItemDataBound(object sender, DataGridItemEventArgs e)
    {
        decimal somma = 20.50m;
        TextBox TbSomma;
        switch (e.Item.ItemType)
        {
            case ListItemType.Footer:
                TbSomma = e.Item.FindControl("Tb1") as TextBox;
                TbSomma.Text = somma.ToString("C");
                break;
        }
    }