This is a question that has been asked by many steemians and answered.
For example:
@bitgeek/how-much-is-my-voting-power-worth
@steemboys/upvote-calculator-what-is-your-upvote-worth-with-x-amount
Upvote Worth Calculator: http://www.steemdollar.com/dollar_per_vote.php?name=steemit (replace the name: "steemit" with the account to calculate).
I have been advised that this is accurate and the above are not:
For example: