You're thinking retail my friend. If you bought 1000 sets of EMG pups you'd get them for around twelve bucks apiece. That's a fact. A preamp? There are preamps in $200 guitars. They can't cost more than five or six bucks to make. So one that's super high quality , I'd say around $50 wholesale in bulk. That's hardly half of $500.
I am not "thinking retail" in any way. You are comparing apples to oranges.
Big difference between the "$12 EMGs" in a Korean Spector and what goes into Spector's American made basses. Same as the Duncans in my Fodera being completely different than the Duncans in my $299 Squier.
Talk to Mike Pope, Nordstrand, Sadowsky, Demeter or Haz Lab to see if they will sell you 1000 preamps for $5-6 each.
The majority of the cost of a $10,000-ish bass is absolutely labour. But I completely disagree that the highest quality "parts" can be sourced for $400-500 per bass.... regardless of the volume.