12 Month XBOX Live GOLD - $36

Cheaper at Boxed Deal by a dollar. Instant email delivery, too. 

http://boxeddeal.com/xbox-live-12-month-subscription-code-email-delivery.html

Or CDkeys.com, 31.49 and I've always gotten my codes pretty quickly from them.

http://www.cdkeys.com/xbox-live/memberships/12-month-xbox-live-gold-membership-xbox-360

Boxed deal is great code appears in account even before email

This eBay seller appears to be gray market (at best) like some of the others mentioned in this thread. My code was emailed and already used. I'm hopeful this will work out OK, but I didn't want to support shady practices.
Moving forward I'm assuming the source of any sub-$40 Gold cards is questionable.