I think that the 645D is about as low a value as it'll get - it has some advantages over 35mm, but really can't drop much more. If you have FA lenses, they're holding their value quite well. If you can hold off 6 months, you may be able to get a GFX at a discount compared to the current pricing, plus when Sony revisions the 50MP chip to 70ish MP, there will be a large group of folks stepping up, no matter the platform.
It's amazing, 3 years of 50MP CMOS 33x44 ...