Tony, The differences between the D700 and D600 are greater than between the D600 and D800 IMHO. All the differences are not good however, like sync speed and max shutter speed. The D700 is also now an older model, some may need to upgrade due to shutter actuation limits etc.
My logic also only goes so far. The most cost effective camera is likely the D3200. I would guess that in reality most of us would be very happy with images obtained from this camera. In > 95 % of situations we would not be able to differentiate an 11x15 print made with this camera vs a D800 or D4. (I know if iso is 6400 or higher the differences would be fairly apparent) Handling of this camera compared to these others however everyone would notice. The D3200 with an 18-55mm kit lens is only $650 on Amazon.
"Cameras and lenses are simply tools to place our unique vision on film. Concentrate on equipment and you'll take technically good photographs. Concentrate on seeing the light's magic colors and your images will stir the soul." Jack Dykinga