The euros and the usa basses all use the same necks. That's why there's not much of a difference but better finishes on the top woods.
There's a lot more than that. From inlays, pre's, hardware, and the US basses are full slabs ( most) not tops. The new euro's are sandwich which actually costs more but still half the bass is solid figured. Hard to call it a top really. Are the US basses 2x as good? Not really imo but that's from the under models being of such high quality to start. But that's the way Spector does things.