Thom Hogan did an analysis of the sensor cost for FX plus a DX body and came to the conclusion that the D600 would be priced around $2000. Almost the only incremental cost is the sensor, and the FX sensor is nearly twice the size of the DX sensor. When you apply normal profit margins, the incremental sensor size alone is what is driving the higher price.
For a lower price, you need radical changes in the camera. Mirrorless is one such strategy so you might see a lower priced FX camera in a mirrorless design - but that is a distant speculation.