Not just any 500 votes will earn you $6k, the payout is based on the the steem power the accounts have.

So if someone went and spent the time to create said 500 accounts and vote for themselves they might only make a buck for themselves. However because it would be pretty clear that they are trying to cheat the system it would only take a couple of downvotes from moderately steam powered accounts to negate every single upvote they gave themselves.