Without going through the printed parts in the farmbot there are cases where PETG is just not a suitable replacement to ABS. Higher flexibility of PETG compared ABS could be a problem in some parts where how ridged ABS is happens to be important and the fact you cannot use PETG as a support material like you can with ABS because its not snapping off.
NuclearWinterIsComin the plates part of the frame you really cannot skip out on being aluminium while your most of your frame is aluminium because you want items to expand and contract at fairly much the same speeds. Its like the other post suggesting of using stainless steel for the plates same basic problem. Even plywood suggested by Josh is going to be less dependable. Also you are going to run into that pure PETG will be too flexible. Josh PVC and gardens is not exactly great due to leaking some chloride that argue with likes of plant protection mesh when it strong enough and PVC for plates would not be strong enough.
The reality is PVC, PET and ABS are insanely weak compared to aluminium. You are looking at optimistically at least 10 times thicker starts seaming insane and that is what you are looking at to get the structural strength in the pure materials of PVC, PET, ABS. Of course carbon fibre filament could be used todo the plates but that stuff is not cheap or fun to work with particularly when you are needing UV stable so ABS or PETG carbon fibre filament. I don’t know if PETG carbon fibre would work out any cheaper than cutting aluminium sheet and still having to argue with the thermal expand difference between PETG carbon firber and the aluminium frame bars. Basically I don’t see it as wise to look at changing the plates in isolation to completely reworking the frame. Material of frame and Material of plates has to be compatible with each other if they not bad things will happen over time.
On the topic of other options to possible reduce cost.
The cheaper home cnc uses just plain box and angle materials to make up the frame. This would require going threw and pricing both ways. So far I have not seen anyone experiment doing a farmbot limiting themselves to the cheaper materials but harder to work with at times materials. Yes box and angle materials means getting your mounting holes placed in exactly right places where the current extruded aluminium in design you can adjust.
Something else to remember the Nema 17 and the Ramps controller the farmbot uses are very close standard reprap 3d printer parts.(the ramps is) So 3d printer built the right way becomes spare parts source for farmbot or vice vs.