## Simple Price Calculator

I am trying to learn Javascript (I'm a PHP developer) and I've written a simple script that I believe is most of the way to what I need.

Its very basic, I have 3 option inputs, first is quantity then 2 add-ons which both default to 0. Then a subtotal would populate beneath based on user input. Default is first option of quantity (25 in this case).

I defined arrays for the 3 option inputs, then assigned a variable to each one via getElementbyID ... then I would simply add them together and here I know my syntax is off. (It's the syntax of JS that I haven't been able to get my head around)

If someone can give me a quick logic guideline as to how and WHY ... that's what I'm looking for. Thx.

My code is below:

HTML Code:
<script>
var card_qty = new Array();
card_qty["25"]=65.00;
card_qty["50"]=120.00;
card_qty["75"]=150.00;

var liners_qty = new Array();
liners_qty["0"]=0.00;
liners_qty["25"]=12.00;
liners_qty["50"]=20.00;
liners_qty["75"]=30.00;

var qty = document.getElementById('card_qty')
var liners = document.getElementById('liners_qty')

function calculatesubtotal() {}
var subtotal = qty + ret_add + liners; // this is clearly wrong!!

var amount = document.getElementById('subtotal');
amount.style.display='block';
amount.innerHTML = "Subtotal: \$"  ++subtotal ++; // and how to reference the subtotal dynamcally is my other question ...

}
</script>

<div id="price_calculator">
<h2>PRICE CALCULATOR</h2>
<label>Choose Qty:
<select id="qty" name="qty">
<option value="25">25@\$65.00</option>
<option value="50">50@\$120.00</option>
<option value="75">75@\$170.00</option>
</select></label><br>
<select id="option1qty" name="option1_qty">
<option value="0"></option>
<option value="25">25@\$12.00</option>
<option value="50">50@\$22.00</option>
<option value="75">75@\$30.00</option>
</select></label><br>
<select id="option2qty" name="option2_qty">
<option value="0"></option>
<option value="25">25@\$25.00</option>
<option value="50">50@\$50.00</option>
<option value="75">75@\$75.00</option>
</select></label><br><br>
<h5>Subtotal: <span id="subtotal">\$101.00</span></h5>
</div>