ok so i have this form on my web page.

HTML Code:
<div id="purhcaseForm">
	 <form id="purchase">
		 <br> Item Price: $463.50
		 <br> Please Select Quantity
		 <input type='button' name='subtract' onclick='javascript: document.getElementById("qty").value--;' value='-'/>
		 <input type='button' name='add' onclick='javascript: document.getElementById("qty").value++;' value='+'/>
		 <input type='text' name='qty' id='qty' />
what i want to do is in the simplest way possible is to have another box which outputs the total order price taking into account the quantity the user has specified. im very new to java script and do not have any clue as to where to start ( i dont even know if what i have is the best way of achieving my objective) all help is apreciated