use template field for data conversion like this
<asp:templatefield headertext="Bill Amount">
<itemtemplate>
<asp:label id="lblBillAmt" runat="server" text="<%#Convert.ToDouble( Eval("BillAmt")).ToString("#0.00")%>">
</itemtemplate>
<edittemplate>
<asp:textbox id="txtBillAmt" runat="server" text="<%#Convert.ToDouble( Eval("BillAmt")).ToString("#0.00")%>">
</edittemplate>
<itemstyle horizontalalign="Right" />
<asp:templatefield>