The sale is busted on 360 at the moment, at least for me. A fair amount of the items on sale have the discounted price listed in the Rock Band store, but when the system-wide "Confirm Purchase" menu pops up, the original price is listed.
For example, Legend and Are You Experienced? have the correct sale price of 800 points on the purchase menu, but Axis: Bold as Love is listed at 800 points on the Rock Band store, but when it comes time to actually purchase it, the price is still the original 1600 points. The same goes for many individual songs and song packs too.
I would imagine that it will take some time for all of the prices to correct themselves, because Microsoft has never faced a challenge on Xbox Live such as altering the prices on over a thousand items simultaneously. So give it some time to hopefully get all fixed, but until then, buyer beware!