double amonut_ = 2073635.00;
System.Globalization.CultureInfo objInfo = System.Globalization.CultureInfo.GetCultureInfo("hi-IN");
string str = amonut_.ToString("C2", objInfo);
txtAmount.Text = str;
<div>
<asp:TextBox ID="txtAmount" runat="server"></asp:TextBox>
</div>
Output--------------
₹ 20,73,635.00