They are supposed to be random, but everyone knows the drill. The first few are easy and it progressively harder to find the prizes. I've been at 7 on two games for two days now. I have 45 spider boxes in one game, 23 in the other. Which probably means 45 I have crap prizes in one game and...well you know how it is.
Gree really needs to rethink these box events. Most of the boxes I am attempting to open are opening, but they are giving me a little bit of cash or a few experience points. Nothing I would consider spending gems for. And since the odds of my getting an Ebon gem are less than 1 in 20 at this stage of the quest, I am not going to use gems to open boxes that 95% of the time they give me crap. Maybe if they gave some decent units out as boobie prizes I'd consider it.