Prices = probabilities. The probabilities displayed on Polymarket are the midpoint of the bid-ask spread in the orderbook. For example, in this market, the probability of 76% is the midpoint between the 73¢ bid and 78¢ ask. The spread between them is 5¢. If the bid-ask spread is wider than 10¢, the probability is shown as the last traded price. You may not be able to buy shares at the displayed probability / price because there is a bid-ask spread. In the above example, a trader wanting to buy Yes shares would pay 78¢ for up to 165 shares, after which the price would rise to 79¢.