What ISN'T in the box, ah?
I think I've cracked it before it's even released.
It's not something, or anything, or even nothing. It's the thought that you spent money on your curiosity. The box cannot and never will be chiselled open.
Everyone who spends money on it gets a chisel of varying strength to crack open the box. If you buy the 59p iron chisel, you get a hit of strength 10 times that of the base chisel; the £50,000 chisel is 100,000x more powerful than it so your chances of cracking seeing inside are much higher.
Everyone who spends more will be "closer to the money shot" as Molyneux put it - or as I've interpreted it, more self-reflective on how much they were willing to spend on subsiding their curiosity. If you spent a fortune, you're gonna' think pretty bloody hard about what you did, and if it's only a couple of quid; you'll still think about it, just not as much.
It's a Thought Experiment, not a lottery.