A prediction market contract may represent the same economic outcome across multiple venues, but that does not mean it can be traded at the same cost.
Displayed prices are only the starting point. For a sizable order, the final economics also depend on market depth, venue fees, the spread, and whether the order takes or provides liquidity. The price on the screen can end up far from the price paid.
To illustrate the difference, we analyzed live order books from July 28, 2026 for comparable contracts tied to the July Federal Reserve decision. The case study considers a client seeking to buy 50,000 contracts.
At the time of the snapshot, the best displayed asks were:
On the surface, Kalshi appeared to be the least expensive venue. But a 50,000-contract order could not be filled entirely at the top quoted price. As the order consumed the displayed asks, its average execution price increased.
Market depth changes the ranking
The average prices available for a 50,000-contract immediate execution were:
| Venue | Top ask | Avg. price at 50,000 | Slippage |
|---|---|---|---|
| Kalshi | 78.0¢ | 78.11¢ | +0.11¢ |
| Polymarket | 79.4¢ | 79.45¢ | +0.05¢ |
| Polymarket US | 79.0¢ | 82.62¢ | +3.62¢ |
Polymarket US illustrates why the top-of-book quote can be misleading. Although its displayed ask was only one cent above Kalshi's, the available depth was materially different. Filling the full order would have produced an average price of 82.62¢.
The marginal price of the last contract matters just as much as the advertised price of the first. A venue that looks competitive for a small order may become the most expensive route for a large one.
Two decisions set the execution cost
The all-in cost of the trade was determined by two choices:
- Where to route the order. Comparable contracts can trade at different prices across venues, with different depth at each price level.
- How to execute it. Taking displayed liquidity crosses the spread and can trigger taker fees. Working an order passively can improve the fill price and replace taker economics with maker economics, although passive fills are not guaranteed.
1. Smart order routing
Smart order routing evaluates the executable price across venues, not simply the best displayed quote. For larger orders, this means analyzing the full order book and allocating contracts to the lowest-cost available liquidity.
In this snapshot, the difference between the least and most expensive immediate venue routes was more than four cents per contract, before accounting for maker-taker fee schedules.
2. Passive execution
Rather than crossing the spread for the entire order, an execution algorithm can work the order over time and seek passive fills. In the case study, the River scenario assumed an average passive fill of 77.00¢.
The comparison with taking 50,000 contracts on Kalshi, the best single venue for an immediate fill, was:
| Cost component | Kalshi taker | River routed, maker |
|---|---|---|
| Average fill price | 78.11¢ | 77.00¢ |
| Venue fees | $598 | $155 |
| River fee | n/a | $53 |
| All-in cost | $39,653 | $38,708 |
The benefit came from more than one source. The passive execution assumption improved the average fill price by 1.11¢ per contract, while maker economics reduced venue fees by 74%. The River fee was included in the final comparison.
What does a prediction market trade actually cost?
Focusing on a broker fee or venue fee in isolation can lead to the wrong conclusion. The relevant measure is total implementation cost:
In the case study, the all-in outcomes were:
| Execution path | All-in cost | Difference vs. River |
|---|---|---|
| Polymarket US, taker | $41,741 | +$3,033 / +726 bps |
| Polymarket, taker | $40,133 | +$1,425 / +355 bps |
| Kalshi, taker | $39,653 | +$945 / +238 bps |
| River: routed, maker execution | $38,708 | Baseline |
After adding River's fee, the routed passive approach still had the lowest modeled all-in cost. Comparing fee schedules alone would have missed most of the difference.
What traders should measure
The same economic outcome can trade on several venues at once. Choosing a venue is only part of the trade; size, depth, urgency, and order type determine the realized price.
For trading teams, hedge funds, and other professional market participants, that means asking:
- Is the contract economically equivalent across venues?
- How much size is actually available at each price level?
- What happens to the average price as the order scales?
- Which maker or taker fees apply?
- Can the order be worked over time without compromising the trading objective?
- What is the final all-in cost after every fee?
The best path may involve splitting an order across venues, changing execution style, or waiting for passive liquidity. The relevant result is the realized price after every cost.