From our friends from gree
"Here is how the special event chances work: Once an event box is opened, one of several things will be awarded: money, respect, an extra grunt/ally, a weapon or unit, or the event special item. Regardless of how the box is opened, the reward is randomly chosen; opening a box does not guarantee that the event special item will be awarded. Each special event item gained will reduce the chance of getting the next, although this chance never becomes zero or below zero."
Make note of the final line. If the regular drop rate is 2% for the tenth, then maybe it is even lower if you have won in the past. I've been successful in the past three 10 box events, which tells me that going forward, I should never use gold if my chance of getting the next is now worse. Don't quite understand why you would want to do this gree, but many things don't make sense....
