The current value of STEEM is about a third of SBD. I always get 3/4s of the number in SBD and 1/4th SP.
That is coincidence. If you select 50 / 50, it is 50 / 50.
However, my explanation above doesn't make much sense, now that I have read it again ...
If the payout shows 10 $ after taking off the curator's cut, then you will get:
5 US $ worth of SP
5 SBD (+some extra because some witnesses publish a 'biased' feed at the moment)