Help me auto update a field in a webpage based on the value of another field

I need to auto update a total amount due field that is tied to MySQL database through php.
User enters quantity, defaul value is set for price each and total due should update on entering quantity and updating quantity or price each.

I asked a similar question yesterday but realize I did not state my goal clearly.

I currently have code where you must click in the total due field to update it and I want to eliminate this.
Click to update appears in red as default value in total due thanks to an answer I got yesterday but again, I want to eliminate this for a more sophisticated form.

Below is the existing code

<tr>
			<td colspan="2" height="23"><font face="Verdana" style="font-size: 9pt">How many attendees are you paying for?</font></td>
			<td height="23" style="border-left-width: 1px; border-right-width: 1px; border-top-width: 1px; border-bottom: 1px solid #C0C0C0">   	
			<p align="center">   	
			<input id="field1" size="4" name="Attendee_Count" value="<%=Request.Form("Attendee_Count")%>" style="border: 1px solid #FFFFFF; padding-left: 4px; padding-right: 4px; padding-top: 1px; padding-bottom: 1px; text-align:right; float:right"></font></td>
		</tr>
		</table>
<table border="0" width="100%" cellpadding="0" style="border-collapse: collapse" id="table13" height="26">
		<tr>
			<td width="7%" height="26">&nbsp;</td>
			<td width="51%">&nbsp;</td>
			<td width="28%" height="26">
		<font face="Verdana" style="font-size: 9pt">Cost Each</td>
			<td width="13%" height="26" style="border-left-width: 1px; border-right-width: 1px; border-top-width: 1px; border-bottom: 1px solid #C0C0C0">
		<font face="Arial" size="1">
			<%
Reg_Fee = trim(Request.Form("Reg_Fee"))
If  Reg_Fee = "" then
   Reg_Fee  = "275"
end if
%>
<input id="field2" value="<%=Reg_Fee%>" size="4" name="Reg_Fee" style="border:1px solid #FFFFFF; float: right; padding-left:4px; padding-right:4px; padding-top:1px; padding-bottom:1px"></font></td>
		</tr>
		</table>
 
	<table border="0" width="100%" cellpadding="0" style="border-collapse: collapse" id="table14">
		<tr>
			<td width="7%" height="31">&nbsp;</td>
			<td width="51%" height="31">&nbsp;</td>
			<td width="28%" height="31">
			<font face="Verdana" style="font-size: 9pt; font-weight:700">Total Due</td>
<style>
.normalButton{
color: black;
}
.redButton{
color: red;
font-size: 6pt;
}
</style>
			<td height="31" style="border-left-width: 1px; border-right-width: 1px; border-top-width: 1px; border-bottom: 1px solid #C0C0C0">
<%
Total_Due = trim(Request.Form("Total_Due"))
If  Total_Due = "" then
   Total_Due  = "Click to Update"
  end if
%>
 
	<input id="Total_Due" class="redButton" value=<%=Total_Due%> onfocus="document.getElementById('Total_Due').value=document.getElementById('field1').value*document.getElementById('field2').value;document.getElementById('Total_Due').className='normalButton';" size="4" name="Total_Due" style="border: 1px solid #FFFFFF; padding-left: 4px; padding-right: 4px; padding-top: 1px; padding-bottom: 1px; font-weight:700; float:right" ></font>
	</td>

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