The event chart shows an outcome at 51¢, you buy, and the fill comes in at 52.9¢. Nothing went wrong. The chart shows the midpoint between the best buy and sell orders, while your trade fills against the actual offers in the order book. Here’s how that works and how to avoid overpaying.
What the order book is
The order book is the list of orders on a market. On one side are sellers (asks): “I’ll sell ‘Yes’ shares at 52¢, 200 available”. On the other are buyers (bids): “I’ll buy at 50¢”. The best ask is always above the best bid, and the gap between them is the spread.
On Polymarket, an outcome’s price comes from these orders rather than from a single quote. Prop Hub trades on the same prices: we read the Polymarket order book at the moment of your trade and fill your order against it.
How a buy fills
A buy on Prop Hub works like a market order on Polymarket. The order takes sellers’ offers starting from the cheapest and works upward until it reaches the amount you entered. The fill price is the weighted average across every level it took.
Example. The order book for “Yes”:
| Price | Shares available | Level value |
|---|---|---|
| 52¢ | 200 | $104 |
| 53¢ | 300 | $159 |
| 55¢ | 1,000 | $550 |
A $300 buy takes the whole first level ($104), the whole second level ($159) and another $37 from the third. That’s about 567 shares at an average of 52.9¢. The best price was 52¢, but the average came in almost a cent higher — that’s slippage.
Selling mirrors this: the order fills against buyers’ bids, from the best price down.
No partial fills
If the book doesn’t hold enough offers for the full amount, the order doesn’t go through at all. In the example above, all three levels together hold $813 of offers. A $900 buy will be rejected, and the panel shows why. The fix is simple: lower the amount.
This saves you from ending up with an $813 position at a worse average when you planned for $900.
Why PnL is negative right after you buy
An open position is valued at the market midpoint, halfway between the best bid and the best ask — 51¢ in our example. At that price your 567 shares are worth about $289, though you paid $300. The account immediately shows about −$11 unrealized, plus the fee.
That’s the normal cost of entering the market, not a valuation error. If the price rises, the position moves into profit. If you hold the outcome to resolution, the spread no longer matters: a winning share is worth $1, a losing one $0.
The fee comes on top
Prop Hub charges exactly the fee Polymarket charges on that market. Each market has its own rate, and some charge nothing. As a share of the purchase, the fee equals the market rate × (1 − price): higher on cheap outcomes, lower on outcomes near $1. On a buy it’s charged on top of the amount; on a sale it’s taken from the proceeds. There’s no fee when a position pays out at resolution.
Across our trades the average fee is about 2.8% of the amount. Worked examples are in the help center under execution and fees.
Sports: a short delay before the fill
On sports markets Polymarket fills orders with a small delay, usually around a second. It protects market makers: otherwise someone could buy at a stale price a moment after a goal, before the orders were pulled.
Prop Hub does the same. After you click “Buy”, we wait out the market’s delay, read the order book again and fill at the fresh prices. If the price moved during that second, the trade fills at the new one. If the book emptied, the order is rejected. More on trading during a match in the article on live sports markets.
How to pay less
- Watch volume. On big markets — a league final, a Fed decision, the day’s headline matches — the book is deep, and even a buy of a few hundred dollars barely moves the price.
- Split your entry over time, not by size. If the book is thin, buy part now and part later, once sellers post new offers. Splitting to get around the per-outcome limit doesn’t work: the limit counts the total of all buys, as covered in the article on risk per trade.
- Compare the button price with the chart. The buy button shows the price you can actually get in at. If it’s well above the chart, the spread is wide and entry is expensive.
- Skip empty markets. If an outcome has no offers, the button shows “–” and you can’t buy at that moment.
- Plan the exit. Selling before resolution also goes through the book. Getting out of a thin market is expensive, so a large position there is often better held to resolution.
