Getting reliable numbers is really complicated. Looking into the wallet history of a vote buyer I see how much he/she has spent. Eventually there's a partly refund because not all of the bid could be filled. There are lot's of invalid bids and refunds are partly coming from other accounts. It's not always the author account buying the votes and more and more memos are encrypted. I don't know the exact ROI, so if the bid was done shortly after the post was created, there may be several other non-bought/front-running/... votes on the post as well. Additionally, users can keep their balance on smartsteem and minnowbooster, so they can buy upvotes without an SBD/STEEM transfer on the blockchain. One thing that may give a clue on the total number is the amount of STEEM/SBD payed to delegators each day, but that's again a (partly user configurable) fraction of the bid, not necessarily of the vote value (unknown ROI...).
MinnowBooster has an API that shows the bought votes - that might give some insights, but that would be only one of several sell-your-vote services.
I didn't even try to get an estimate on this, so I really can't give any number here...