Sadly, because it's a digital download, we don't know exactly how many copies of Lost & Damned Microsoft and Rockstar managed to sell. We do know, however, it's more than 323,000.
Microsoft's Aaron Greenberg let slip the information in an interview with GameDaily - in which he calls the Grand Theft Auto IV expansion "the most successful game addon content we've ever launched" - saying "If that content was sold at retail, it would be one of the best sellers across all platforms. It would have outsold Killzone 2".
Interesting. Not for the "outsold" argument (which is hardly fair, since L&D was out for half of February while Killzone 2 was only out for a few days), but for the Lost & Damned sales ballpark. Nice to get one for a downloadable game, even if it still leaves us wondering...