For technical details, see my PiDP-8 project. In terms of electronics and software, the PiDP-11 is not much more than a variation of the PiDP-8's design.
The PiDP-11 will be a bit more costly to make, because of the planned custom case. Design goal is to stay well under $250 though. If done well, the molds for switches and case will be useful for other PDP-11 projects as well. The idea is to have them available as 'open-source molds'. If such a concept exists. :)