Updating Cost Automatically

Hi guys,
I'm trying to create a simple quote calculator to show the user how much tickets will cost if they choose X quantity of tickets. I'd like the quote to update in the #total_figure div each time they change the quantity from the option selections.
For example, if they choose 2 tickets at $50 and 1 ticket @ $20, the #total_figure should be $120
Many thanks for any help!!

Code (text):

<form id="book" method="post">

<label>

   <span class="bold_yellow">Ticket 1 @ $20</span>

     <select…

Updating Cost Automatically

See original post by martin12532