For resale that is. If I am going to purchase a computer with the student deal with an iPod, only to turn around on eBay and sell it.... Which would be easier to sell / hold its value better?
The 30GB model has been a very popular model, and has probably outsold the 8GB Nano, making the nano more exclusive. Also, the nano has flash memory as opposed to a hard drive, which makes it more durable, thus lasting longer.
Not everyone needs 30GB of media in their pockets, as well.