I got some statistics that might help you out to take the decision of staying in a certain league of not:

• Average value of Bronce chests: 0.02$
• Average value of Silver chests: 0.058$
• Average value of Gold chests: 0.18$

So, in order to get more value in Bronce than Silver you should be getting more than x3 times the chests in Bronce. Example: If you get 20 chest in Silver you should get >60 in Bronce.

Same thing with the difference between Silver and Gold!