There's an interesting quote on Joystiq from a GDC Europe talk by BioWare director of online development Fernando Melo, who says that a majority of the profit they saw from Dragon Age DLC was from sales of day-one DLC packs Stone Prisoner and Warden's Keep, saying this accounted for 53% of their DLC income from the game (thanks nin). Word is:

This figure accounts for those who bought Stone Prisoner for $15 in a used copy of Origins, and those who purchased Warden's Keep for $7 outside of its inclusion in the Digital Deluxe Edition. Since these two DLC packs, Dragon Age: Origins has had seven additional iterations of downloadable content, which account for less than half of the total DLC revenue from Origins.