"These" JDF devices ? Some JDF "devices" are simply software to process JDF files. For example, our PressWise system has a option to generate a JDF file that is was developed to pass certain printing related details for a job to a JDF Processor that expresses someting very simple - like what paper to select from which tray, where to fetch the imposed PDF file to print with (it is a URL in the JDF file ) and how many copies to print. That is it. We don't charge "extra" for that. I don't think HP charges extra for the Indigo DFE to have the JDF Processor either.
But if we are thinking "what about a cutting or folding machine" - then the "extra" cost might be - developing a JDF and JMF software ( to process files and messages ) - which requires some physical or wireless connectivity to a network and some computer to process this data and run the machine. So, added costs would be a computer, a network connectivity system ( EtherNet or WiFi ) and software development.