Althought 32 bit chips would probably have been more expensive that 16 bit chips, at least as cent/bit is concerned, the total amount of RAM would affect the price more that width of the chips.
I can't find a hires picture of an Atari falcon motherboard, but the schematic suggests they use 4bit ram (which is likely for the time). You put 4 next to each other for 16bit, 8 next to each other for 32bit. If you use the same size ram then for 32bit you either have double the ram, or you halve the size of each chip. Either way having 16bit access to ram is considerably cheaper than 32 bit access.
However, you need a more sofisticated RAM controller and custom chips for 32 bit memory access. Maybe the design decision was made because they had 16 bits designs for custom chips etc. that could easily be added to the Falcon (reuse=lower development cost)? I would call that a lazy approach;)
They could have kept 16 bit custom chips (pretty much like AGA did), but the memory controller would need to be changed. It's no more complex, just more lines. They would need to do another spin of the hardware and relayout the motherboard. If they had changed to 32bit after development began then it's unlikely that they'd have ever shipped anything.
I found this development document for AAA. It looks pretty neat on paper and much, much better than AGA. It was cancelled in 1993 in favour of more advanced architectures. Probably a good thing by -93. But imagine an Amiga with this architeture in 89-91. Awesome!
http://www.thule.no/haynie/research/nyx/docs/AAA.pdf
It would have been too expensive. They weren't considering selling it in A500/A1200 type systems. It was when someone finally remembered that commodore was supposed to be selling high volume of cheap hardware that it got cancelled.
I'd take AGA with 16 bit chunky and a texture mapper in the blitter over AAA any day (texture mapping is not much more complex than line drawing). The problem was that nobody at commodore had the vision that cheap crappy 3d rendering would be such a big deal.