I have no idea about the base amount for monthly bonus as Platinum 3 users, but what I received every month is always more than $100, so maybe the base amount is $100. 
My wagering stats every month is around $50k-$100k but I never tried to calculate it with profit stats.
Here is what I received in the last 4 months: June = $130, July = $120, August = $125, September = $110
I did not wager much in the last 30 days, less than the average amount I did during June to August so I got less amount this month than the other 3 previous months.