Unattended Robots usually take over much longer, more demanding process, then attended bots in my experience. Attended robots help the user’s speed when performing their duties in the form of tasks. It can make a great assistant, but a benefit from the solutions is going to depend on the user’s relationship with the robot.
Unattended Robots, on the other hand, have the potential to run 24 hours. The returns of this and benefits I can see much greater because the robot is potential handling whole process with little intervention open the chance to push much more benefit out of this robot then the attended. This is based on what I have encountered. So to tie it all together, to answer your question, the price difference is due to ROI in the form of benefits, and even the demand of the Unattended Robots.