More expensive because there is less demand? Doesn't that usually go the other way?
Am I mistaken in thinking that they outsource to other people? (I'm having trouble loading their website due to the load.)
Also, doesn't HN rank upvotes based on karma and account age? You can't downvote until reaching 500(?) and having your account for a certain duration of time.
That sounds about right.
There seem to be some additional rules, too. For instance, there are certain comments I don't see a downvote button for, and I have no clue why.
So in essence what you have is a seller determining which type of the two accounts to make. He will not be able to re-use the HN accounts as much, and therefore he makes less per unit of time from them, and must charge more to make them worth his time.