/* this is the magic code that allows js to be put within the head tag and only execute only when the dom is fully loaded */
var onDOMReady = function(f) {
 var ol = window.onload;
 if(window.onabort != 'function') {
  window.onload = f;
 } else {
  if(ol) {
   ol();
  }
  f();
 }
}

// how to use it :
onDOMReady(function() {
 // place all your js within the function wrapper here 
 alert("this only execute after the DOM IS FULLY LOADED SO NO ELEMENTS WOULD BE NULL.");
});
/* if you don't want to use this function wrapper move all js to just before the end of </body> it will produce the same effect */

Amount 1 :
    <asp:TextBox ID="txt_a1" runat="server"></asp:TextBox>
    <br />
    Amount 2 :
    <asp:TextBox ID="txt_a2" runat="server"></asp:TextBox>
    <br />
    <asp:Button ID="btn_calc" runat="server" onclick="btn_calc_Click" 
        Text="Calculate" />
    <br />
    <asp:Label ID="lbl_answer" runat="server" ViewStateMode="Enabled"></asp:Label>
    <br />

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btn_calc_Click(object sender, EventArgs e)
    {
        // init ours ints
        int a, b = 0;
        try
        {
            // parse text as integer
            a = int.Parse(txt_a1.Text);
            b = int.Parse(txt_a2.Text);
        }
        catch (Exception ex)
        {
            // if parsing fails send error msg to label
            lbl_answer.Text = "Error : Numeric values only.";
            // end script execution
            return;
        }
        // finish calculation and post it to label
        lbl_answer.Text = (a * b).ToString();
        // end script execution
        return;
    }

}

/// <summary>
    /// Re-usable functions that splits an amount into s[0] = currency symbol and s[1] = decimal value as string[]
    /// </summary>
    /// <param name="ammount">input a string like $100.30</param>
    /// <returns></returns>
    private string[] RenderAmmount(string ammount)
    {
        string[] s = { ammount.Substring(0, 1), ammount.Substring(1, ammount.Length - 1) };
        return s;
    }

protected void Page_Load(object sender, EventArgs e)
    {
        // step 1 :  the values I need to work with
        string a = "$100.12";
        string b = "$2.35"; 

        // step 2 :  i need to split those values into amounts I can use in a calculation
        string[] a_rendered = RenderAmmount(a); 
        string[] b_rendered = RenderAmmount(b); 

        // step 3 : the calculation
        double d = Math.Round(double.Parse(a_rendered[1]) * double.Parse(b_rendered[1]), 2, MidpointRounding.ToEven);

        // step 4 : I need to reconstruct the string into its preferred format
        string answer = a_rendered[0] + d.ToString(); // we are using the currency value from our first render and the caluclated amount
       
        // output the string which is $235.28
        Response.Write(answer);
        
    }

<table width="100%" border="1" cellpadding="0" bgcolor="FFFFFF">
    <tr style="font-size:14"><th>ITEM</th> <th>QTY.</th> <th>PART NO.</th> <th>DESCRIPTION</th> <th>NC/NR</th> <th>PRICE</th> <th>TOTAL AMOUNT</th></tr>
    <tr>
        <td>    
            <div align="center"><asp:TextBox runat="server" name="itm2" ID="itm2" MaxLength="2" Width="40px"></asp:TextBox></div>
        </td>
        <td>
            <div align="center"><asp:TextBox runat="server" name="qty100" ID="qty100" 
                    MaxLength="7" Width="70px" style="text-align:right"></asp:TextBox></div>
        </td>    
        <td>
            <div align="center"><asp:TextBox runat="server" name="prt2" ID="prt2" MaxLength="25" Width="175px"></asp:TextBox></div>
        </td>
        <td>
            <div align="center"><asp:TextBox runat="server" name="remark2" ID="remark2" MaxLength="36" Width="240px"></asp:TextBox></div>
        </td>
        <td>
            <div align="center"><asp:TextBox runat="server" name="Nr2" ID="Nr2" MaxLength="3" Width="45px"></asp:TextBox></div>
        </td>
        <td>
            <div align="center">
                <asp:TextBox runat="server" name="price100" ID="price100" 
                    MaxLength="10" Width="55px" style='text-align:right'
                    OnTextChanged="price100_TextChanged" AutoPostBack="true"></asp:TextBox></div>
        </td>            
        <td>
            <div align="center"><asp:TextBox runat="server" name="amount100" ID="amount100" MaxLength="14" Width="120px" style='text-align:right'></asp:TextBox></div>
        </td>

<tr>
        <td colspan="5"></td>
        <td align="center">
            <asp:Label runat="server" name="grndTotalLabel" ID="grndTotalLabel"><b>GRAND TOTAL</b></asp:Label>
        </td>
        <td>
            <div align="center"><asp:TextBox runat="server" name="grndtotal" ID="grndtotal" 
                    MaxLength="14" Width="120px" style="text-align:right"></asp:TextBox></div>
        </td>
    </tr>

protected void price100_TextChanged(object sender, EventArgs e)
    { 
        decimal result;
        result = Convert.ToDecimal(qty100.Text) * Convert.ToDecimal(price100.Text);
        amount100.Text = "$" + result.ToString();
        grndtotal.Text = "$" + result.ToString();

        itm3.Focus();
    }

//I'm going to need to add "YOUR" code into each of the Price??_TextChanged Event so it gets fired at the exact moment I need it to.

protected void Page_Load(object sender, EventArgs e)    
{
        // step 1 :  the values I need to work with        
        //string a = "$100.12";
        string a = amount100.Text;

        //string b = "$2.35";
        string b = amount101.Text;

        // step 2 :  i need to split those values into amounts I can use in a calculation        
        string[] a_rendered = RenderAmmount(a);
        string[] b_rendered = RenderAmmount(b);

        // step 3 : the calculation        
        double d = Math.Round(double.Parse(a_rendered[1]) * double.Parse(b_rendered[1]), 2, MidpointRounding.ToEven);

        // step 4 : I need to reconstruct the string into its preferred format        
        string answer = a_rendered[0] + d.ToString();

        // we are using the currency value from our first render and the caluclated amount
        // output the string which is $235.28
        Response.Write(answer);
}