The cost shouldn't be 0% or 100%, but the percentage chance of getting the reward, multiplied by the value of the reward. IOW, E(X) = P(X) * X, so a 50% of 100K is effectively a 50K payout. The problem is P(X) is unknown, and you only really have heuristics to guesstimate. Whether you move forward with the idea 0%/100% is just if you are a glass half empty/full kind of person, but the real answer is somewhere in between.

From looking at tldroptions.io, it's just taking an empirical approach to guess the P(X) value.