A prediction market card may show 63%, but that does not guarantee that a buyer can enter at 63¢ or that a seller can exit there. The headline probability is a useful summary of the market. Execution happens inside an order book, where available prices, order size, and platform costs determine the real trade.
The distinction matters whenever you compare opportunities: a better displayed probability can still produce a worse net trade if the market is expensive or difficult to execute.
The quote attracts attention. The spread, depth, slippage, and fees determine what the quote is worth to you.
The displayed price is a starting point
Interfaces need one number to represent a market. Depending on the platform, that number may reflect the latest trade, a midpoint, the best available offer, or another display convention. Each can be informative, but none replaces the prices and quantities currently available to trade.
Bid and ask describe the executable edges
- Best bid: the highest price a buyer is currently offering.
- Best ask: the lowest price a seller is currently willing to accept.
- Last trade: the price of the most recent completed transaction, which may no longer be available.
- Midpoint: the value halfway between the best bid and best ask, even though no order may exist exactly there.
The basic relationship is spread = best ask − best bid. A smaller spread usually means the market offers a tighter path between entering and exiting.
Spread is the first execution cost
Suppose the best bid is 60¢ and the best ask is 64¢. A screen might summarize the market near 62%, but an immediate buyer pays 64¢ while an immediate seller receives 60¢. The four-cent gap is not necessarily a fee charged by the platform. It is the distance between available buying and selling interest.
That distance becomes especially visible in a quick round trip. Buying at the ask and immediately selling at the bid would lock in a loss before any explicit fee. A patient limit order may improve the entry price, but it introduces a different risk: the market can move away before the order fills.
A wide spread is information too
A wide spread can signal limited participation, disagreement about fair value, uncertainty about settlement, or simply a quiet moment. It does not prove that the displayed probability is wrong. It shows that turning that probability into a trade may be costly or uncertain.
Fees change the break-even point
Platforms use different fee schedules and can apply costs at different stages. A trader should review the current rules for the specific venue and contract instead of assuming that a quoted price is the complete cost. Even a small charge can matter when the expected edge between two markets is narrow.
Costs to check before trading
- Trading fees associated with entering, reducing, or closing a position.
- Settlement or profit-based charges that may apply when a contract resolves.
- Funding, withdrawal, conversion, or network costs outside the order itself.
- Minimum charges or rounding rules that can have a larger percentage effect on small trades.
For comparison, think in terms of net cost = fill cost + all applicable fees, then compare that figure with the contract’s possible payout.
Slippage appears when size meets limited depth
The best ask only describes the first available quantity. If a buyer wants more contracts than are offered at that level, the remaining order continues into higher asks. The average fill price moves above the first quote. That difference between the expected price and the average executed price is slippage.
- The order first consumes contracts available at the best ask.
- Any remaining quantity moves to the next price level in the book.
- The process repeats until the order is filled or available liquidity runs out.
- The weighted average of those fills becomes the effective entry price.
Order size is relative to the market
A trade that is tiny in one market can be large in another. Slippage therefore cannot be judged from order value alone. It must be compared with the quantity resting near the current price. This is why order-book depth is often more useful than a single liquidity label.
Market orders and limit orders trade different risks
- An immediately executable order prioritizes completion, but the final price can move through several levels.
- A limit order controls the worst acceptable price, but it may fill partially or not at all.
- Posting liquidity can improve price discipline, but exposes the trader to changing information while waiting.
There is no universally best order type. The choice depends on whether price certainty or execution certainty matters more for the trade.
Compare markets using executable net prices
A cross-platform gap is actionable only after the contracts are equivalent and the trade survives its costs. Start with the quantities you actually intend to trade. Estimate the average fill on each venue, add relevant fees, and consider the cost or uncertainty of moving funds and exiting later.
Use the probabilities in the markets comparison grid as a discovery signal, then inspect each source order book before treating the displayed gap as available value.
A pre-trade execution checklist
- Confirm what the displayed probability represents on that platform.
- Record the best bid, best ask, spread, and available quantity at nearby levels.
- Estimate the weighted average fill for your intended order size.
- Add every applicable trading, settlement, funding, and withdrawal cost.
- Decide whether a limit order, immediate order, or smaller staged trade fits the situation.
- Recalculate the expected return using the executable net price—not the headline quote.
Execution turns a probability into a position
Prediction-market prices are excellent discovery tools, but trading begins where the summary number ends. Spread measures the distance between buyers and sellers. Depth determines how much can trade near the quote. Slippage captures the effect of size, and fees determine what remains after the platform’s costs. Read together, they reveal whether an apparent opportunity is still attractive when it becomes a real position.
