While originally said to be DLC for New Super Mario Bros I, New Super Luigi U will be released both as a downloadable add-on and as a special standalone version that will be available at retail. The retail version will not require a copy of New Super Mario Bros. U to play.
Suggested price is $19.99 for the DLC and $29.99 for retail, multiplayer will be enabled in the game and will make Nabbit a playable character.
New Super Luigi U will be released on June 20th in North America and Europe on the Nintendo store. The standalone packaged version will be released on July 27th in Europe.
Iwata confirmed that there are 80+ new courses replacing the original courses from New Super Mario Bros. U