According to video game Analyst Michael Pachter, Microsoft has sold near the 1 Million mark for The Lost And Damned DLC bringing in around $20 Million in revenue. Pachter also goes on to predict that the total sold may double to 2 Million by October making the total $40m, only $10m short of the $50 Million Microsoft paid for exclusivity.
Add to that the fact that there is another DLC set to release this year and future deals that can include greatest hits and bundle deals in the future and this turns out to be a very lucrative deal for the folks at Microsoft.
UPDATE: Two ways to look at this. Failure or Success. If you look at it from a sales standpoint it is only 1/7th of the users who bought GTAIV. If you look at it in terms of the revenue, Microsoft already recouped almost half of their investment with another DLC on the way that could put up the same numbers.