As I recall New Vegas shipped ~5million copies (tracked November last year that's ~$300million) and the 360 version made up the bulk of the sales
(1) for the previous month (at least in the states). I have no reason to believe it didn't track in similar proportions the rest of it's US life. I have no idea what proportion of the total sales were converted into DLC sales, but I can see MS covering the potential lost DLC sales in co-marketing/cash/lower sales take per unit easily.
If it didn't make economical sense to them, they wouldn't do it. Still bullshit though.