You could probably improvise something cheaper using the same filter arrangement, though now that you have it, sounds like the best thing to do might be just including an extra sacrificial initial filter to increase the lifespan of the proper ones.
Were you maybe thinking some kind of wire mesh, like the "washable pre-filters" in air purifiers tend to be? I did initially consider this; but, after running the air scrubber for a few months and seeing what actually builds up in the pre-filter, it seems like the pre-filters don't actually end up being coated in anything (dust/hair/etc) that can actually be rubbed/scratched away or even vacuumed out of them. Rather, just end up gradually blackening (due, I think, to fine [PM2.5] particulate making it partway through them before gradually embedding deep inside them); and gradually feeling more and more "skudgy" to the touch (due perhaps to some of those particulates being hygroscopic, and due to also trapping things like pollen/mold spores.) You can actually touch the surface of the fouled pre-filter and your fingertip will come away clean (but touching it won't be a nice sensory experience.)
I think, for helping with that kind of particulate, a wire mesh wouldn't do much if anything. The only "passive" thing that would help is a non-woven cross-linked fiber material, porous enough to not overwork the air scrubber's fan to the point of making it work harder / be louder / burn out sooner, yet layered and "internally labyrinthine" enough to capture particulate despite that porosity. I.e. the same material used in the pre-filter.
(If I could add an active component, I might introduce an intake-side ionizing air purifier, where particulate gets charged and then preferentially deposits onto the fins of a washable heatsink-like grille. AFAIK, those don't normally work at the sort of CFM being pushed by the air scrubber. But if anyone wants to design a better mousetrap here, that might be a direction to look into.)