Weird... I just has the similar thoughts last night.
From my limited understanding on the economics of cameras, the main difference in costs between APS-C vs FF cameras are due to the sensor yield. Since camera sensors are always fixed in their dimensions, unlike other chips, they do not benefit from Moore's Law. The only time when costs go down is when you can get more usable dies out of each wafer. The big boys in semiconductor manufacturing (Intel, TSMC, etc) are pushing migration from 300mm to 450mm wafers. 450mm fabs are supposed to be coming online next few years.
The last migration from 200mm to 300mm occurred in 2000 with reduction in price of die by 30% to 40%. Once 450mm migration is complete, we should see a gradual drop in sensor costs for FF and MF. APS-C costs will also decrease but I supposed not as significant in overall terms as it take up a smaller percentage compared to cost of the rest of the body. This is a relatively slow process that will probably be drawn out for the next 7-8 years.
As price of FF drops to current APS-C levels, cameras from the lower end (the PnS, mirrorless, compacts, even your mobile phones, etc) are also moving up in terms of performance and/or size so where does that leave the APS-C?
Interestingly, Canon's choice of having EOS-M having same sensor size as their current crop cameras opens the possibility that Canon will eventually replace the EF-S with EF-M. Having the same sensor size smoothens this transition and all current crop DSLRs could be replaced by mirrorless versions in the long run. (Perhaps we'll get the 17-55 f/2.0 in a EF-M mount?) This is also Canon's answer to the 4/3s? It could be the end of EF-S, but not the APS-C format.
The other question is why are we seeing entry level FF ahead of 450mm migration. I'm not sure why. Perhaps somebody better informed care to enlighten? My only guess is both Canon and Nikon already foresee this happening and trying to get the first mover advantage. Being the 2 leaders in this field, they have the resources and is in their interests to capture this "new" market. Afterall, moving from crop to FF usually includes a change in lens system and the APS-C user is not really tied down to their current brand. It makes sense to create an established base of entry level FF users early than trying to compete as a late comer when the price of FFs really starts falling.
Well, all this is just pure conjecture. Intellectual masturbation, if you wanna call it. ;p