Second machining the amp panels out of a chunk of aluminum rather than pouring the metal into a mold, on a budget amp! Don't know how much more expensive it is, but I think more work goes into the former process.
Looks like the panels are simply cut and CNC'd from a thick Al sheet. The milling is not particularly extensive... only two maybe three bits used. All things considered, not that expensive as your costs are basically just machine shop time.
Casting molten metal into that shape would be much more costly. Initial investment would be very high in the creation of the molds. Heating metal up to molten state is also relatively expensive (consider this: it is sometimes cheaper to transport molten metal from the refinery to the manufacturing/casting plant because it costs so much to reheat and melt). Casting quality in a thin sheet is also quite terrible and you can have porosity or warping or slag tainting the material. Even if you could get the casting to come out relatively clean and uniform (more $$), the crystal structure is gonna be crapola and inconsistent because the molten material that hits the surface first cools very quickly while the remainder cools more slowly. You can fix that by with post heat treatments, again more $$. Finally, you'll have to actually finish the surface, so you're paying machine shop time anyways.
Die casting would be better/more consistent quality (basically a metal injection mold), but that initial setup cost is even higher. Drop forging or similar could be cheaper in huge quantities, but you'd still need to machine the holes.