Payout (in $) shouldn't be influenced by price of steem, I guess. For example your payout is 5$, what means 2.5SBD and 2.5 USD (in steem). It's 2.5SBDx2.5USD (it's current price of steem) + 2.5USD in steem, which is 8.75USD. When prices decrease then payout is still 5$, but now when price of steem and SBD is for example 1USD, it's 2.5SBDx1USD and 2.5USD in steem, so it's only 5 in USD.
Payout (in $) shouldn't be influenced by price of steem, I guess. For example your payout is 5$, what means 2.5SBD and 2.5 USD (in steem). It's 2.5SBDx2.5USD (it's current price of steem) + 2.5USD in steem, which is 8.75USD. When prices decrease then payout is still 5$, but now when price of steem and SBD is for example 1USD, it's 2.5SBDx1USD and 2.5USD in steem, so it's only 5 in USD.
But payout is still 2.5SBD + 2.5USD (in steem)...