Actual post value is measured in STEEM, and the value shown is the value of your post in USD. It's calculated by multiplying the STEEM value of your post by the USD value of STEEM.
The payout assumes that 1 SBD = $1 for now - witnesses are able to change this. This means that, like you say, if you pick a 50/50 rewards split the payout in USD is much higher than what is shown under a post. If you pick to power up 100%, the reward is spot on (ignoring the curator cut).