Last month I visited a quadrotor laboratory ($ 20m^2$ approx). They used both Optitrack and Vicon systems. However, they told me that Vicon cost 10 times as much as Optitrack. Considering that any system of motion capture relies on the same principles of IR markers, which might be the reason for this difference in price?
This comes mostly from two things, the hardware and the software composing these systems.
The hardware, mostly what are capable of the 'cameras' used.
The second one is the software, that include calibration algorithms (different speed of calibration and accuracy), reconstruction algorithms (how many marker per object, accuracy of the body reconstruction), exportation format (how many bodies, single markers, in which format).
It boils down that Optitrack is good enough for UAV flight, but that over activities have different requirement which are met best by Vicon.