According to Harmonix on their website, “The Rock Band 3 export will be available for $14.99 for people that are verified purchasers of Rock Band 3 only. We will be automatically verifying your purchase electronically, which means that you’ll need to have bought a new copy and played Rock Band 3 for our export process to work.”
The developers also note that they are approaching the expiry date of when they can sell Rock Band 3 on the Xbox Live Marketplace which is set for the 30th of October, so if you want a digital copy of the game you will need to purchase it by the end of this month. However alternatively you can also purchase a physical copy from retailers if they are still carrying units in stock.
This isn’t the first time Harmonix has introduced backwards compatibility. In previous versions of the Rock Band series, gamers could export songs from the previous title. However it seems that prices have gone up from $5, to $10, and now it costs $15.