Theoretically the .arduboy format being worked on allows for different .hex contained in one. So it is possible to package the same game together (if cross-compiled and included) that will run on both systems.
The chip is cheaper but this new setup will need a bit more passives, which does increase the cost some, but not by much. The design will likely require a 4 layer pcb which also adds to the cost. We would want to include an SD card with games installed.
Need to check, but it's my estimate any cost we recover from cheaper components we will end up spending on the SD card, but maybe not.
Of note on the cost savings side, I'm actually looking at a snap-together design. This will reduce the need for the screws, which are actually very cheap... But it will completely remove the need for screws on the assembly line which really speeds up production.
We will need a new mold for the front and back for this.
So, all in all I would guess the cost could more or less be kept the same. At the end of the day, the volume of units has a much greater influence on the cost of the units to produce. So keep letting us know what you would like to see in the new one!