I'd venture to guess that the take rate on custom configs from any manufacturer is quite low. Most consumers are going to go to Best Buy, Apple Store, et al and buy what config is available today, not wait to order some custom model.

Then it's a good thing no one charges that. I'm all for calling out Apple for bullshit, but their "768 GB more" model is a $200 upsell, not $500. And yes: it's a huge leap from $200 to $500.