It would be interesting to know the minimum of had to have been gambled to have got to each level, including 8. That would be a good basis on which to objectively evaluate them. There are other factors of course, but that would be a good start.
I think minimum risked/gambled would be really hard to define because so many of the games have varying payouts and the goals are usually in "amount won" not in "amount risked". For example, if you walk up to a fire game there's a chance that you get a huge jackpot on the first bet, so you would have risked very little (what's the minimum, IDK, but you'd have to know the highest possible jackpot at the lowest possible stakes to evaluate it).
There's probably a good way to do it in terms of expected amount risked. That is, if all the games are (say) .95EV and you need to win 100BTC to get to level N, then we could say that on average we expect people to risk 105.26BTC(=100BTC/.95) to get to that level.
I think that's basically the closest you could come to caculating risk for a given level. That is, use expected, rather than minimum.
