In a lab you can demonstrate how much material something uses, how long it lasts under given conditions, and, say, how sensitive it is to contamination. But you can't predict, you know, the late-oughties polysilicon price bubble, the ensuing long-term purchase agreements made by companies like Evergreen, and the subsequent polysilicon price collapse that sunk those companies. (I'm still not sure where modern PV cell companies source their silicon! Is it UMG?)
"How cheap?" is a question about international trade, mining, and management, not a question about materials science. Don't expect materials scientists to be able to answer it.