I was searching for this earlier and i think @yuriks2000 is much more informative, however, i found a link to: http://www.steemdollar.com/dollar_per_vote.php?
I have no idea how accurate it is because it was posted last year-- I found it on @mauricemikkers page after a n in-site search:
https://steemit.com/steem/@mauricemikkers/steemdollar-com-calculating-the-worth-of-a-single-upvote-what-is-your-upvote-worth-in-usd
Just put in the @user and it calculates it for you.

Cheers! Thanks for the additional source!