The servo they use is $493[0], at that price I wouldn't necessarily consider it a hobby servo. The control for that specific series elastic servo needs some work. There have been other attempts at making cheap series elastic actuators. An interesting one was the programmable spring work[2][3]. Although one problem with series elastic actuators is that they can be difficult to control because of the compliance. Force servos were also an interesting attempt at doing cheap force control[4] and by using load cells they avoided the compliance problem. Unfortunately, force servos did not have any position control.