How about concrete? You can get the stuff they make concrete countertops out of at a store near me for around $15/bag. Past a point it seems it would make more sense to have portable forms and pour the box onsite. Or have the box made out of interlocking vertical layers that stacked on top of one another. How inert and resonance free would a box be if it were brick and you just had a brick layer lay it in your listening room? It wouldn't cost much. Maybe have a light box and cover it in a thick layer of concrete. If performance were really the point of these ridiculous boxes there are bound to be a lot of better ways to going about building an inert box than spending 120 hours machining aluminum.
The thing is, past a point it can't possibly make sense to have the entire speaker delivered to a house. You can do a lot of things for 100k, let alone 500k, that you can't for 10k. It's a very different situation and calls for a different approach. Or it would, if performance were the primary consideration.
