Tom-
There are a number of commercial companies who offer 'builders assistance'. Most of these require the 'builder' to come to their facility where they get 'assistance' on putting the parts together. On the RV-6 kit planes a builder could essentially pay someone to bring it up to quick build status (Fuselage 80% done, Wings 99% done). I wouldn't assume the DAR would even blink an eye at that. On my paperwork I specified that I had a quick build RV-10.
They don't offer a QB option on the RV-6 but they do on the RV-7 which is similar in nature for an $11,175 up charge. Put about $1800 for shipping (savings) and a decent ball park number to bring it to this stage would be $13K. If I had to take a shot in the dark I would say a professional, or someone who has built an RV before could do this in maybe 600 to 800 hours. The remaining 51% as people keep pointing out is sucked up by the rest of the kits; empennage, finishing kit, firewall forward and final assembly. Then you have the engine, avionics and interior installation. Knowing the time-sink I got into with all these I would guess you would spend at least another 600 - 800 hours doing these for an RV-6.
So in the end I would give the owner the a la carte option. $13K or more to bring it up to the QB status. $2K to build the tail-cone. The rest I have no clue on, would probably be in the owners best interest to participate in that so he can answer any questions the DAR has on the building process or what was done and why. For the naysayers I have yet to hear of any case where an Airworthiness Certificate was denied because the DAR felt it wasn't 51% amateur built. I have however heard of one case where the 'builder' had some issues getting the repairman cert because the FSDO felt he didn't build the plane.