What is normal nuetral to ground resistance on a duplex receptacle, 120 volt 20 amp circuit in an office setting?

There is no "normal resistance", as it will change due to length of conductors, reliabilty of connections, etc.

This is not a valid value, since the only connection between both are at the "single point of coupling" of a service.

Why do you ask?

Assuming everything is wire nutted properly and grounded at the box and that no one has switched a hot for a neutral, it should read zero or close to it. Unless this is a relatively new install, neutral and ground are on the same bar in the box. Even if you have separate neutral and ground bars, it should still read zero. Copper wire has great conductivity and unless you are running a long distance it will not be increased that much. Standard wire gauge tables show 12AWG at <2 ohms per 1000 ft.