We're a small children's theatre company and make great use of Google Analytics. We use CiviCRM/CiviEvent for our patron database and online ticket sales, and wanted to add Google Analytics ecommerce tracking to track the effectiveness of our various marketing activities.
I did this by copying /civicrm/templates/CRM/Price/Page/LineItem.tpl into a custom template directory, and adding the tracking code to my custom version. All of our "events" use price sets, and the price options are various ticket categories (child, adult, senior, etc). The code below tracks not only the entire order amount, but also the quantities of individual items in the order.
LineItem.tpl is called multiple times during the order/checkout process, so my custom template inserts the tracking code only when the $trxn_id variable is present -- this indicates that the order is finalized.
If you are not using price sets for your events, then you'll need to copy and modify a different template. Probably /civicrm/templates/CRM/Event/Form/Registration/ThankYou.tpl.
Patch available here: http://forum.civicrm.org/index.php/topic,19039.0.html