Sorry, but full frame will never be inexpensive or common.
The reason is CMOS manufactuing technology. One 8-inch silicon wafer can hold either 200 APS-C sensor or just 20 full frame sensors.
If there are 15 errors on the wafer, the wafer yields either 5 full frame or 185 APS-C dies.
Because of the large chip size and low yield, the full frame sensor will always be substantially more expensive.
This is according to a Canon White Paper.
-Paul