The money would certainly be spent elsewhere, but the point is "why burn precious energy to bring a rock here from China if I can find a rock in my front lawn". Do what you like with the money you save, it is still a win for the planet because you conserved a resource at no cost (if the two rocks are fungible)
Also it occurs to me that while a focus on local may cost gains found in economies of scale, it can net gains in terms of spreading out the burden of negative externalities. This one is murky and not always a clear win one way or the other though.
It is not at no cost, because it is more efficient( at least comparatively if not absolutely ) to produce the rock in China and overseas shipping is extremely efficient. That is what comparative advantage and trade is all about. It is only better for the planet in the sense that the products costs more and as a result fewer of them will be produced costing less resources.
Also it occurs to me that while a focus on local may cost gains found in economies of scale, it can net gains in terms of spreading out the burden of negative externalities. This one is murky and not always a clear win one way or the other though.