I expect the mkIII to be more expensive than the D800, too. Canon can afford that, they are still the 1. in sales. Also, if you price something higher people think it's better. :-$
The probable reason why the 5DMkII prices held higher than the D700 was its unwavering consumer demand. Demand for most cameras would have slowed to almost non-existent after maybe about 10-12months. But that didn't seem to be the case with the 5DmkII. After the photographers, came waves and waves of videographers. The D700 had its MSRP slashed sometime back because of demand drops I think. It was very close in price to the 5DmkII at launch as far as I remembered.