Problem Solved :-)
<asp:TemplateField HeaderText="Balance" ControlStyle-Width="3%">
<ItemTemplate>
<asp:Literal
ID="Literal4"
runat="server"
Text='<%# (Decimal.Parse(Eval("totalamount").ToString())-Decimal.Parse(Eval("paidamount").ToString())).ToString("N2") %>'>
</asp:Literal>
</ItemTemplate>
</asp:TemplateField>