Keep in mind the timeframe. Five years ago FF was much more expensive to manufacture than it is today. The fact that the 6D, still relatively new, sells for as little as $1700, is quite telling here.
It seems that Canon's fab procedure hasn't changed, at least from what we can tell. Has the cost of silicon wafers come down that much? Or could it be that Canon was reaping very high profit margins on the presumed high cost of a FF sensor, and now they've decided to push more units at a lower profit margin, as an alternative strategy to drive the bottom line?
Just sayin'. Not that I'm cynical, or anything...
I do believe 300mm wafers have come down in cost. I remember them still having some challenges five to six years ago with defect rates on them (this is pretty agnostic of industry...not specific to sensor fabrication). It is obviously a less serious problem for tiny chips like GPUs and CPUs or other ICs. Growing the wafer crystals has become more refined over the years, in no small part to some of the advancements made while trying to perfect the process for growing 450mm wafer crystals (which, as far as I know, has still not been taken up by any IC manufacturing industry...there is apparently a very high initial cost to jumping that has to be recouped, something no manufacturer seems willing to deal with as of yet).
As far as I know, it's not that actual growing of the Silicon ingots to 300mm (or 450mm for that matter) that is the yield/cost issue. Getting very high quality and uniform ingots has always been extremely important to high-voltage devices (think kilo-Volts), because spikes in dopant concentration can result in avalanche-type catastrophic breakdowns (leading to the magic blue smoke leaving the devices in spectacular ways).
It's the yield of the manufacturing processes for everything you on the base wafer that matter (diffusion, implants, etching, photo lithography, metallization etc).
The problem for the jump to 450mm wafers is that you need the entire infrastructure of a fab to be in place: steppers, aligners, photo lithography, and so on. It's so expensive to develop, that basically the entire IC industry needs to coordinate around it; it's not enough that Intel or TSMC (or both) says "hey, we want to do 450mm now" and presto! they have it.