> or example you had early Scottish shortbread primarily using rice flour because it was cheaper at that time.

What era of history are we talking about here? Would it have been transported as flour, or ground locally?

While I don't have the exact details of transport and whatnot, much of that rice came from the Carolinas https://inlandrice.charlestoncounty.org/economics.html

Generally speaking, rice is a lot higher yield per acre, and also, the east coast doesn't have particularly good wheat growing.