This is the cheapest server to run a witness node on 
Means 340~ STEEM per month. To be able to cover this cost with witness rewards you need to be at least inside top50.
Being a witness doesn't have a simple formula like that example. We're here to support the network and nobody promises any guaranteed rewards for running a witness node.
However, I see some of the witnesses already thinking to disable their node because of the constant negative ROI, that may be a good time for you to cast your all witness votes.
Steemit page to vote: https://steemit.com/~witnesses
A wider witness list: https://steemian.info/witnesses
Self-plug: I am also a witness for a while. You can cast a witness vote for me from here with a click if you didn't do already.