The base chance is 0.08%, which is 1/1250.
If you bought 1250 keys just to hit the 'drop rate' - you would also have 105 guaranteed purples (including the second key you use in this event which unlocks a purple).
The chance of receiving a Zombie Walk Token from that guaranteed purple isn't known but it would sound fairly reasonable to be around 1/1000 and makes it a little easier to predict how many tokens could have entered the game this weekend.
There's no way of knowing how many keys were used this weekend, but if it was 1m -5m keys in total, we're looking at 1k - 5k tokens released this weekend. Most of which have been consumed already.
31-Oct-2021 18:11:10