There are lots of other people in here who know a lot more about this stuff than I do, but I suspect there are a few things that contribute to the price of subwoofers that make them look disproportionately more expensive than other types of speakers.
One is that most have their own internal amplifiers. Essentially, you're getting something that most speakers don't have in them: electronics.
Another thing is that subs are, typically, a lot bigger than other types of speakers. Simply put: more materials, more expenses.
The construction of the drivers, too, to generate the low end frequencies probably contributes as well. I gotta believe that while all speakers require some clever engineering, there's gotta be something about subwoofers and the acoustic challenges they present that make them more expensive.
And while it's not part of the cost of manufacturing, due to their weight, shipping for subwoofers is probably substantially more expensive than a normal speaker.
Just my thoughts.
