As @littleboy said it: the ROI is most of the time around 10%, even with bid-bots. The only difference is that you can gamify the timing of the vote, while with @smartmarket from Smartsteem the timing is always dynamic, which means you get 10% (or what the current ROI is at the time) however old the post might be.

With that said: creating posts just for the sake of buying votes is not appreciated in the Steem community and will results in flags.

Instead, invest time into writing creative, honest posts/contributions and promote them with a reasonable amount.

Additionally, you can either sell your votes with Smartsteem or delegate to Smartsteem to earn ROI, while the first option gives you the freedom to vote manually as well and not having your Steempower locked.