I've completed the schematic for the CPU portion of the design. It's a pretty big design - I'll post it here shortly. There's 21 chips just for the CPU. And just placing them on a PCB, it's about half the size of a standard piece of paper, but routing might make this bigger. I have a 16-bit address bus and an 8-bit data bus to route everywhere, so I will almost certainly have to do a 4-layer board. Estimating the cost of this - from EasyEDA this size of a 4-layer board is about $46+$8 shipping for 5.
Ok, I just checked if I double the size of the board (for the peripherals, clock, power) the price goes to $59+$8. So I may just put everything on a single board. It would cost a lot more to make two boards of the same size of a single large board. I'll still have an expansion bus for debugging and possible future projects (video out maybe?) I'll just have to make sure I get it right the first time. But then, it's only $59+$8 to rev it, right?