Complain about the cost of Call of Duty map packs all you want; it doesn't stop Activision charging a ton for them, and it's certainly not stopping people from buying them.
The publisher revealed today that over 20 million map packs for the Call of Duty franchise have now been sold. That's not just Modern Warfare, it's for the entire franchise, but boy oh boy, 20 million. That's a lot, and given the relative recency of downloadable content, you'd have to imagine most of those are derived from the two Modern Warfare games (though Activision did not discloses an individual breakdown).
Some may say that accounts for 20 million suckers (or 5-10 million suckers who are repeat offenders), and on a good day I'd be prone to agree, but really, nobody forced them all to buy it. It just goes to show for all the wailing and gnashing of teeth that goes on in gaming forums and websites, there are untold masses out there perfectly content with paying $15 for a handful of maps.