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You're suuure that was p/t? That's about what we pay up here for a sheet of 3/4" exterior-grade ply, and a quick google suggests the price is about the same in the States. Exterior-grade is not pressure-treated, it just uses waterproof glues to laminate the plies together (as opposed to regular, non exterior grade whose glue will disintegrate faster in the weather.)
There is supposedly such a thing as true pressure-treated plywood, where the wood plies are pressure-treated as well as the glue being waterproof, but I have to say that when I lived in upstate NY and now living in Ontario I never did manage to track down a source for such a thing (for other purposes). As PC says, heaven knows what it'd cost. There is marine-grade plywood but I am not sure whether that is pressure-treated, I *think* (someone correct me) it is just a higher grade of plywood with waterproof glue? Marine-grade is pretty expensive.
Anyhow, you should definitely use exterior-grade plywood for a coop (unless you can get the other kind for free and the coop doesn't have to last real long). But p/t plywood is certainly not necessary in any way shape or form, even if you can find and afford it.
Good luck,
Pat