How to ajax this dropdown php variable - php

Question

I simply don't get jquery or ajax.
I understand javascript just fine, but for some reason, I can't ajax through my head.
I'm sorry I have to ask this, but can some help me, I just spent literally the last day trying to make this jquery/ajax work for this drop down box, been through a bunch of examples. Tried them. Don't work.
This is what I'd like.
The below drop box to dynamically change a variable in php without a submit button.
I would like to use only one file, a php file named single-event.php .
then display $total php variable in that h2 everytime the drop down box is changed.
<?php $cost = 10.00;?>
<select id = "attendee" class='postform' onchange="Onchange(this)" >
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
</select>
<?php $total=$cost*$(drop_down_box_selected); ?>
<h2>
<?php _e('Submit a payment of $' . $total , 'pippin_stripe'); ?>
</h2>
I normally wouldn't go this low and simply ask someone to do my work but I'm just sick of trying, this was the closest i got to anything i wanted, but discovered I NEED to convert the value to a php variable.
Please help, I'm basically pulling my hair(not really as I'm bald), tired and still have an entire different project that needs to be finished by Monday.
<select id = "attendee" class='postform' onchange="Onchange(this)" >
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
</select>
<script type="text/javascript">
var dropdown = document.getElementById("attendee");
function onCatChange() {
if ( dropdown.options[dropdown.selectedIndex].value != -1 ) {
document.getElementById("hi").innerHTML = dropdown.options[dropdown.selectedIndex].value;
var price = document.getElementById("hto").value
var num = dropdown.options[dropdown.selectedIndex].value;
var total = price*num ;
dropdown.onchange = onCatChange;
</script>
Thank you to anyone who gives any help.