Aluminum LA block??
The only aluminum small block engines I remember seeing for sale were designed for Sprint and Drag racing. There a bit removed from the stock block and require some retardedly expensive parts instead of making use of stock parts over again. While, I am sure theres a good reason for that, it does leave us out of the picture.
As far as the 3rd gen HEMI goes in that it should be an aluminum block out from the factory and installed in the car like other makers do, I'd love it, but expensive it probably is. To make things worse for the #3 of the big 3 is volume. It's more expensive when your not making them just to make'em cause you now your gonna sell them anyway.
Heads are expensive as well. This is why the old 1 gen HEMI engines were drop in favor of a cheaper, small and lighter Wedge head of the B/RB series engines.