I think it is nearly impossible to compare price of 5DIII and d800. beside the cost of producing the camera, there are many othere causes we don't even know about to bring up cost.
Least but not last, d800 has a newly developed sensor, with AF and meccanical part allready used in d3, d700, d3x, d4, . 5DIII is a completly new camera, with a new sensor, new AF, new body. This may be one of the reason of a higher price for the camera.
d800 has a new sensor, new body, updated af and metering used in d4, shutter life @20,0000
5d3 has a new sensor(maybe updated), new body, new af from 1dx, metering from 1100d, shutter life @15,0000