Sizing and pricing a prediction market order
What a position costs, pays and can lose; how the bid, ask and spread work; and when market and limit orders fill. Tables, worked numbers, no advice.
Updated November 10, 2026 · Lumina Markets
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Every contract pays $1 or nothing, so the arithmetic of a position is fixed before you place it. What changes from one order to the next is the price you pay, and that comes down to two choices: how many contracts, and whether you take the price on offer or name your own.
What a position costs, pays and can lose
Three numbers describe any position:
- Cost: contracts × price, plus the transaction fee of $0.01 per contract and a commission of 0.35% of contracts × price.
- Payout if right: contracts × $1.00.
- Loss if wrong: everything you paid, contracts plus fee and commission.
The same 100 contracts at two different prices (illustrative):
| Yes at 25¢ | Yes at 75¢ | |
|---|---|---|
| Contracts | 100 | 100 |
| Paid for the contracts | $25.00 | $75.00 |
| Transaction fee | $1.00 | $1.00 |
| Commission | $0.0875 | $0.2625 |
| Pays at settlement if right | $100.00 | $100.00 |
| If right | $73.9125 more than you paid | $23.7375 more than you paid |
| If wrong | You lose all $26.0875 you paid | You lose all $76.2625 you paid |
Statements and confirmations show these amounts rounded to the cent.
A low price costs less and pays more if it's right, because the market puts a low chance on it. A high price costs more and pays less, because the market thinks it likely. Neither is cheap or expensive by itself; the price is the market's estimate of the odds. The payout calculator runs these numbers for any price and size.
The transaction fee is charged by ForecastEx, the exchange every order executes on, and the commission by Lumina. Fees → covers every cost of a trade.
The bid, the ask and the spread
A contract has two prices at any moment. The ask is the lowest price someone will sell at, and the bid is the highest price someone will buy at. You buy at the ask and sell at the bid. The gap between them is the spread.
Take a Fed decision market where the Yes contract on "no change" shows a bid of 74¢ and an ask of 77¢ (illustrative). Buying 10 contracts costs $7.70 plus a $0.10 fee and a $0.02695 commission. Selling them a minute later, with nothing changed, returns $7.40, less another $0.10 fee and a $0.0091 commission. The round trip costs $0.53605: $0.30 of spread, $0.20 of fees and $0.03605 of commission.
Spreads are narrow where many traders are active and wide where few are. Liquidity is the word for how easily you can trade without moving the price, and the market's chart shows volume alongside price so you can tell the two apart.
Market orders
A market order fills right away at the best price available. You enter the amount you want to spend, and the ticket estimates how many contracts that buys at the current ask.
The estimate can differ from the fill. If the ask moves before your order reaches the exchange, or there aren't enough contracts offered at the best price to fill your whole amount, part of the order fills at the next price up. In a thinly traded market that next price can be well above the one you saw. Check the price before confirming, or use a limit order.
Limit orders
A limit order sets the most you'll pay per contract. You enter the number of contracts and a limit price, in 1¢ steps up to 99¢. The order fills only at your price or better, and if it can't fill right away it stays open as a working order.
The limit also caps the cost exactly: 40 contracts at a 30¢ limit can never cost more than $12.00 plus the $0.40 fee and $0.042 commission, however the market moves.
Choosing a limit price
Where you set the limit decides how the order behaves:
It fills right away against the contracts on offer, at the ask or better, the way a market order would, with a ceiling on the price.
It waits until a seller comes down to your price. It may fill in minutes, later, or not at all.
There's no right setting. It's a trade-off between getting filled and the price you fill at, and only you know which matters more for the position.
Partial fills, expiry and canceling
A limit order doesn't have to fill all at once. If 25 of your 40 contracts have traded, you'll see the filled quantity and the average price so far while the other 15 keep working.
When you place a limit order you choose how long it stays open. Good 'til Canceled keeps it working until it fills or you cancel it. The end-of-day option cancels it at the daily session rollover, and the order ticket shows the cutoff.
Working orders sit in Open Orders, where you can cancel them. To change the price, cancel the order and place a new one. A filled order can't be canceled.
The size is your decision
How many contracts to buy is up to you, and the most a position can lose is everything you paid for it. The Kelly criterion is a formula that turns your own probability estimate and a market price into a fraction of your money for one contract; that page works through the math and where it breaks down.
Lumina does not give investment, tax, or legal advice, does not recommend any contract or strategy, and does not assess whether trading event contracts is appropriate for you. All trading is self-directed.
FAQ
Why did my market order cost more per contract than the price I saw?
There weren't enough contracts at the best price to fill the whole amount, or the price moved before the order arrived, so part of it filled at the next price. A limit order prevents that by capping the price.
Does a limit order guarantee my price?
It guarantees you won't pay more than your limit. It doesn't guarantee a fill.
Why do Yes and No add up to slightly more than $1 when I buy?
You buy each side at its ask, and the asks include the spread. The midpoints add up to about $1.