I'll give you a brief summary of my experience. I have met with a couple of plumbers - both master plumbers, licensed in DC.
Best price so far is $6,300 - this is made up of the install of the back flow preventer, along with installation of a sump pump to address the exterior drain. Without re routing the exterior drain (outside my basement front door) and install of the sump pump this drain would overflow from runoff if the backflow preventer were engaged - and thus flood the basement.
So realistically both have to be done. Neither plumber was interested in using a separate contractor for the digging and cement work as they were concerned about inspections, and the ability to provide a guaranty if the work was not performed entirely by their respective companies.
Given the significant non reimbursable expense, I am not sure that I can move forward at this point.