Most of the bots are actually bid on for too much by everyone so they usually result in negative immediate ROI, you can check on steembottracker.com and see their last bid window to verify. Only a few bots have guaranteed positive ROI, but they often have stricter rules for posting.

The main purpose of bots from the consumer standpoint should be to get visibility and hope for others to vote on your post through the quality of the content, so it's kind of a gamble no matter which way you look at it, considering the price of steem or sbd is always fluctuating and could be higher or lower by the time your post pays out.