... do you think prices will drop for mf digital backs in the future?
If anything, I expect the opposite: MF gear will move a bit further up the scale of lower volume and higher price. As 35mm format becomes accepted as "good enough" for some part of what previously required MF, and so some photographers choose 35mm where in years past they would have chosen MF, two things happen:
1. Reduced demand for MF makes the economies of scale worse, so prices go up even for gear with equal unit production costs.
2. MF makers shift their emphasis towards the higher quality part of their product range, to be clearly above what 35mm can match, maybe with things as simple as higher sensor resolution, which also pushes demand for lenses of ever higher resolution, ever more precisely aligned bodies and AF systems, and so on, so the gear becomes more expensive to make too.
We have already seen signs of this, with the prices of lenses and bodies often significantly higher now than in the film era.
P. S. of course any talk of the D800 or even D800E killing off DMF is either wild hyperbole or siliness, if only for only trivial reason: 80>36. That is, along with all the other much debated differences, resolution clearly does also contribute to a good number of camera purchasing choices, as shown by the higher prices of the higher resolution backs.