Even though it may be smaller, the FM8 just simply costs as much to make. In the simplest terms, manufacturing costs do not scale according to size. Another example of this is why PSA has not introduced smaller subs than 15", they would cost just as much for them to build unless they change parts of their operation (which Tom has finally decided to do in order to make a competitive smaller product). Rythmik could increase the price of the L22 so that you pay more for larger. Many other companies would, but Rythmik has chosen not to gouge their customers in that way and are comfortable with their margin on the L22 as is, which makes the L22 an awesome bargain.
The FM8 is designed to be more of a mid bass module than a stand alone subwoofer, though it can be stand alone in a small space with very limited room.