It took me weeks to understand how the Steem half was calculated. Yes, you can't really say that the post value is in SBD - it's really in a "token" with half the value in SBD and half the value in USD (that will get paid in Steem.)
I made a post on that here (with examples) that generated quite a bit of discussion: https://steemit.com/steemit/@theinsideout/i-finally-understand-50-50-steemit-author-rewards-and-so-can-you