Have you heard of something called bid bots? They are vote selling bots with massive amounts of Steem Power delegated to them. Anyone can delegate to a bid bot and get paid for it (The profits are roughly equivalent to what you could get by upvoting only your own posts and nobody else upvoting yours). People who want to promote their posts can send STEEM or SBD to a bid bot put their post URL in the memo and get a large upvote from the bid bot they've sent money to. The size of the upvote depends on the amount of money they send. Bid bots are a promotion tool. It is very difficult and time consuming to turn a profit using them, which means that you generally have to pay about as much as you get in the form of an upvote. Money is made by delegators and the operators of bid bots. 

The largest payouts without paying for votes with the current levels of STEEM and SBD prices have been around $100-$150. It's extremely difficult to get more than $100. It takes having massive amounts of SP of your own, self-upvoting, something exceptionally interesting to say, having bunch of high-SP friends as well as tens of thousands of followers.