Becoming a Whale
For most of the people on the Steemit platform, their goal is to become a whale. The specific goals might be different: buy a car, be financially independent, not have to work 8am-5pm. Regardless, all those goals are more easily realized by having more Steem Power (SP). You'll get there quicker if you have more money at your disposal
Voting Bots
For better or worse, there are voting bots on Steemit and they're rather ubiquitous. Wherever you look, there are bots promising to give you upvotes for set amounts of SBD sent to their account. There are a few types of bots that you may encounter.
Content Promotion bots: provide a predetermined upvote for a specific amount of money. i.e. 300% return bots would give you an upvote that is 3x the amount of money you send them. Send 1 SBD, get a $3.00 upvote in return.
Bidding bots: will distribute at set intervals the maximum value of their vote in proportion to the amount paid. i.e. if your bid was 10% of the total bids, you'll get 10% of the upvote.
People have debated whether the bots are appropriate or if they're some way of gaming the rewards system and stealing from everyone else. The outcome of the debate/discussion/conversation was that most "bidding" bots limit their votes to posts that are less than 3.5 days old. Not all bots adhere to this, but to be 
Bidding Bot Profitability
The profitability of bidding bots is never guaranteed. You bid in the hopes that fewer people will bid on the service than the upvote is worth. Here's an example of a bid that went wildly wrong. Due to overbidding, the vote was worth only 48% of the bids.
Content Promotion Bot Profitability
This is what I really want to focus on. The returns are preset, so it's predictable and you can easily do the math to determine profitability.
Let's start with the prices of Steem and SBD
Source
Many of you have already seen the charts to determine what the payout will look like, but here's another one
Source:
The above chart shows that if your post has $1 of upvotes, you should expect to see approximately $2.81 in actual value added to your account based on the price of Steem and SBD. Now let's look at the PROFIT to get that value if you're using an upvote service.
This chart shows that if you're not getting any curation bonus and therefore only getting 75% of the payout rewards, you will only Break Even if the voting multiple is greater than 2.3, or 230%! So if you're considering a bot that offer 2.1 times whatever you give them, you'll lose money. You would be better off just selling the SBD or exchanging it for Steem (not considering the potential for future gains on either currency).
The BIG Money
Now you know that if you want to become a whale (which you probably do) then don't use an upvote service unless they are offering greater than a 230% return on your investment.
If you found this useful, please consider upvoting and/or resteeming.
Thanks!
Disclaimers: This is not investing advice. It's for informational and/or entertainment purposes only. These calculations do NOT take into consideration the time value of money or the opportunity costs when investing in upvote services. All calculations are my own. What you do with them is your business, not mine.