You can check @lukestokes's report here to get an idea https://steemit.com/exchangereport/@lukestokes/exchange-transfer-report-5-7-2017-to-5-13-2017
Out of the top 50 withdrawals only 3 users are earning author reward @tamim @abdullar @dollarvigilante
The bulk of the money going out is not made by people cashing out their rewards.
If you want precise numbers use exchange's wallet history to calculate how much authors's money is going out VS steem coming in