We haven't had a good deal on a Memory Stick in quite some time. Thankfully, buy.com is ready to help us out with a great deal on a SanDisk 4GB Memory Stick. The Memory Stick is available for $30, with free shipping. For many, 4GB is more than sufficient to store all your games, saves, movies and music. [Update: FRYS.com is also offering a $30 Memory Stick, with free digital movie.]
If you need more space, the best deals we've found for larger Memory Sticks have been on amazon.com. They have a SanDisk 8GB card for $65 shipped [Update: Another 8GB card has been discovered on buy.com for $60], and a Sony 16GB card for $149 shipped.