From 9to5Toys.com:

Buy.com offers the SanDisk Ultra 64GB Class 10 Secure Digital High-Capacity SDXC Card, model no. SDSDU-064G-A11, for $46.99 with free shipping. That’s $0.73/GB and tied as the best price we’ve seen for a 64GB Class 10 SDHC card. (It’s a current price low by $10.) It features read speeds up to 30MB/sec.

With almost every Mac now shipping with a built in SDXC card slot, these speedy, huge capacity Flash storage cards are like microscopic external hard drives. They are great for storing music and movies or even portable backups. If you are looking to hook up your iOS device, SD card readers start at around $5 at Amazon.