Ouch. Proves numbers can go all over the map. Depends on what slate, who's doing it, underlayment, and all that other stuff you already know about. It might be that my local guys are simply bumping up the plastic cost to push folks into going real slate. For me, it's an aesthetic issue. I hate impropriety. Plastic fake slate is somewhere close, or possibly right at, the nadir of impropriety. Right down there with the peel and stick stone siding and vinyl window shutters. I'd rather see pole barn roofing instead of plastic slate. At least it's honest.