It is $800 based on my own experience that I got some time ago + few dollars from my latest gameplay.
I think you played a lot lately so guess you will get nice amount from the additional bonus.
$800 is fair enough if we look at all the given promotions by Stake regularly (weekly and monthly bonus) that you may not find in other casino.
Perhaps we may get more than $1500 for Platinum 3 if there is no other bonuses such as the weekly and monthly bonus.