i think the point is that SD cards are a lot cheaper than you think them to be. The fact that a 2 gig SD card is 7 dollars retail means that:
1) they are very cheap at wholesale (even cheaper than 7 dollars)
2) you are getting ripped off if you are spending more than 10 bucks on an SD card
Most likely, if the DLC content was made offline, any expansion pack or extra songs on Rock Band will be on a disc because even though the cost of a 2 gig SD card is 8 dollars, the cost of a 4.7 gig DVD is maybe 10 cents to produce.