The Xbox 360 USB Drives will release on April 6th. There will be an 8GB and a 16GB version. The suggested retail price for the 8GB is $39.99, and the 16GB will be $69.99.
http://xbox360.ign.com/articles/108/1080560p1.html
http://xbox360.ign.com/articles/108/1080560p1.html