It is directly related to Steem value. Lets say Steem Price is $1 and someone who upvoted you had 5 000 Steem Power. At a price of $1 per Steem this power will generate $0.5 in potential payouts. Now, if steem will increase in price before your payouts occur (within 7 days of initial post) and rises up to 1.2, this means that your potential payouts will also increase and will therefore be 0.6 dollars. If it decreases, the potential payouts will also decrease accordingly. It is called potential, because you never know what will happen during this 7 day pending period.
Today SBD costs 6 times more than Steem. But as long as SBD is pegged to USD, you receive 6 times more dollar value per SBD per payout.
Hope this will help.