Another consideration is availability. Most of the resellers don't actually stock these items, though a few do. They have to drop ship directly from C&M. And, C&M's ability to deliver is dependent on their stock. At the time I called them (I don't remember if it was late 2009 or early 2010) they only had ONE steering box for a 280 SL on the shelf! That could disappear in a heartbeat of course.
I believe there were some situations where they would actually take the core first for a rebuild. Don't know if you'd get that kind of service from some anonymous parts reseller on the internet, however. That was one of the reasons that prompted my call to them.
Just my findings when I was looking...prices did vary widely, too, as George noted.