Not very sure, but you can start charging like 1 DEC per match in Bronze and below,5 DEC in silver ,10 DEC in Gold, 20 DEC in Diamond and 30 DEC in Champions per match if you have crossed the limit of 50 matches in a day.
I do not think any human players 50 match in a day but based on data we can increase the limit of 50.
Whatever DEC is collected by this method ,put back in rewards pool.
I think if we implement system like this, bot will automatically stop playing 24*7.