Perform Simple Division In HTML

I have a product website that lists the product price based on a box of 20 units ($20/box for this example). I’d like to show the per-unit cost next to the box cost, or $1.00.

The HTML already defines [PRICE] as the tag for the price of the box. I thought it would be simple enough to divide this by 20 using a <script>, but I’m wrong!

I came up with this:

<!--START: quantity_items-->
 <tr>
  <td>[lowbound][highbound]</td> //this the volume band; example - buy 3 to 5 boxes and pay a certain price, called [PRICE].
  <td>[PRICE]
   <script>
    var numOne=[PRICE], numTwo=20, res;
    res = numOne/numTwo;
    document.write(" ($" + res + "/unit)");
   </script>
  </td>
 </tr>
<!--END: quantity_items-->

…with the expected outcome being:

3 – 5 || $20.00 ($1/unit)

This is a table, obviously (td and tr tags), so the calculation would repeat for each row.

Now, calculations work using this tutorial (I can see them on my site):

<script>
  var numOne=12, numTwo=20, res;
  res = numOne + numTwo;
  document.write(" ($" + res + "/unit)");
</script>

But when I replace the 12 with [PRICE], it fails. What is happening here? I assume that [PRICE] needs to be introduced into the <script> somehow, but how it that done?