Okay, thanks. You think your initial quote is fairly accurate for the invisibility approach. Another way you could do it is to use a scrying sensor and illusion to project an image on the wood of the other side to fake transparency, but that would only get cheaper for larger surfaces (it scales better as a solution).

You think there might be a way to do it through transmutation that could be cheaper at large scales (high up-front cost to design a spell and likely a high-powered staff to cast it, but then repeatable), but you're not sure how well this would work and it would probably take weeks to months if research to design.