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do you mean with reputation steem power? Steem power is only relevant if you upvote your comment.
to the article rewards, as said currently 25% of the article reward is distributed on the comments.

Maybe this is a dumb question, but... here it goes: sometimes a post has 1 vote and already earned some money (let's say, $0.50). Other times, the post has 6 votes and $0.00. What explains this?

lol, it explains, that in steem one vote is not one vote :)
The payout depends on the steempower the voter has.
also the payout is exponentially not linear. so post with a lot of steempower voted on it get lot more. This is to hinder self voting.