The bid and ask are determined by traders themselves as they assess the probability of the proposition being true or not. In simple terms, if the bid and ask on a binary option are at 85 and 89, respectively, then traders are assuming a very high probability that the outcome of the binary option will be yes, and option will expire worth $100. If the bid and ask are near 50, traders are unsure if the binary will expire at $0 or $100 it's even odds.

If the bid and ask are at 10 and 15, respectively, that indicates traders think there is a high likelihood the option outcome will be no, and expire worth $0. The buyers in this area are willing take the small risk for a big gain. While those selling are willing to take a small but very likely profit for a large risk (relative to their gain).