Just found this on Action Figure Insider and thought I'd post. You guys were absolutely right. Here's the text and the link:
Q: Why are MOTUC and Dark Knight Movie Masters 20.00 on Mattycollector.com when a DCUC figure is less at retail?
A: The answer is simple. When we produce a small run of figures Mattel is charged more by our factories. In the case of Dark Knight Movie Masters, we offered them to online and retail stores but no on was able to commit to a large enough order to go into standard production. Seeing as how cool these figures are, we wanted to get them out to fans and the best solution was to do a smaller run and offer them on MattyCollector.com. We have to charge a bit more due to the smaller production run.
https://www.actionfigureinsider.com/main/mattels-qa-answers-for-august-2009#more-4346