Right now Amazon has Xbox Live Gold subscription cards for $35.99, a discount of $14 off the normal price of $59.99. If you play co-op games online, you'll need an XBL Gold subscription to do so, as it is a requirement of Microsoft's network. The subscription also gives you access to things like Netflix, Hulu, and ESPN 3.
Use this link to order a card and support Co-Optimus in the process. Gold Subscriptions are stackable, so even if your current subscription isn't up, you can take advantage of this pricing.