You look at the 0.55 on the screen, click market buy, and think the cost should be 0.55 dollars. Then you check the result and see your average cost was 0.58 or even 0.6 dollars. The platform did not overcharge you. The rules of a market order mean it only cares about speed, not price.
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The Core Problem: A Market Order Buys the "Current Lowest Available Price," Not the "Price Shown on the Screen"
Let us clear up one concept first. The trading rules in prediction markets are the same as in traditional exchanges. The 0.55 you see on the screen is usually the "best ask price," which is the cheapest sell order currently available. The problem is that if there are only 10 shares for sale at 0.55, and you want to buy 100 shares, the system will take those 10 shares first, then keep buying at 0.56, 0.57, 0.58, and so on until it fills your 100 shares. The final price you get is the average of all these trades, which is naturally higher than the 0.55 you first saw.
Why Is the Actual Cost Higher Than the Displayed Probability?
Reason 1: The order book is not deep enough, and slippage eats the difference
Polymarket uses a central limit order book model. The number of shares available at each price level is limited. If your market buy order is larger than the total amount available at the best ask price, the system will "eat up" deeper, more expensive price levels.
It is like going to a vegetable market. If the cheapest seller only has 10 pounds, and you need 50 pounds, you have to buy from the more expensive sellers too. On average, you will pay more than the cheapest seller's listed price.
This is especially bad in markets with poor liquidity. Some markets on Polymarket have very wide bid-ask spreads, sometimes more than 5 cents. If you use a market order in such a market, you are essentially jumping into the spread and trading at the most expensive price.
Reason 2: Platform fees are deducted separately, not included in the displayed price
Polymarket's market order fee is deducted directly from your shares, not calculated separately from the payment amount. When the share price is very low, this problem becomes much worse.
Here is a real example. A user once bought 100 shares in an "Economy" market at 0.001 dollars per share. The platform fee was calculated as 0.0948 dollars. Because the share price was so low, this fee was equal to 94.8 shares. The user only received 5.2 shares, and the potential profit shrank from 100 dollars to 5.2 dollars. The effective fee rate was 94.8%. This extreme case was caused by a formula bug that has since been fixed, but it shows a key point: the market order fee is calculated as a percentage of the trade amount, not built into the price you see. The cheaper the shares you buy, the higher the fee is as a percentage of your shares.
Reason 3: The execution mechanism has "slippage tolerance"
Polymarket market orders also have two execution modes: FOK (Fill or Kill, meaning the entire order must be filled or it is canceled) and FAK (Fill and Kill, meaning fill as much as possible and cancel the rest). If you choose FOK, the system will try to fill your entire order at once. To do so, it may have to take prices from 0.55 all the way up to 0.60. If you do not set a slippage tolerance in advance, your fill price could be several percentage points higher than expected.
Risk warning: In markets with wide spreads and thin order books, the difference between the actual fill price of a market order and the displayed price may be far beyond your expectation. The 0.55 probability you see only represents "the price of the cheapest single share right now," not "the price for buying 100 shares."
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How to Avoid Overpaying
Case A: If you are not in a hurry
Switch to a limit order. A limit order lets you set the maximum price you are willing to pay, so you do not have to worry about slippage eating your profits. Limit orders are also maker orders, so the platform does not charge a fee and may even give you a rebate.
Case B: If you are in a hurry
Take 10 seconds to check the order book depth before placing your order. Look at the L2 order book data and see how many shares are available at the current best ask price. If the amount you want to buy exceeds that number, either reduce your order size or mentally adjust your expected cost upward.
Case C: If you have already overpaid
Do not use market orders next time. In prediction markets, a market order is essentially a tool for "accepting the worst current market price." Do not use market orders for large positions just because they are convenient. It is not a tool suitable for large trades.
