Why a prediction market’s headline price is not your execution price

A last-traded price describes a completed trade. A best ask describes the lowest current offer to sell, for the quantity available there. A best bid describes the highest current offer to buy. None by itself tells you what a larger order will cost, and an interface may use a midpoint or another calculation for its headline probability.

Before comparing two screenshots, identify which number each is displaying and when it was recorded. A chart point can remain unchanged while the current order book moves. Comparing a stale last trade with another venue’s current ask measures different things.

Consider an illustrative order book: the last trade was $0.50, sellers offer 20 contracts at $0.51 and another 80 at $0.55. Buying all 100 costs 20 × $0.51 + 80 × $0.55 = $54.20 before fees. The weighted average price is $0.542, not $0.50 and not the best ask of $0.51.

If each winning contract settles at $1, the maximum gross proceeds are $100. The difference between proceeds and the purchase cost is $45.80 before fees. Treating the headline price as the fill would overstate that difference by $4.20. These are invented prices that isolate the effect of order-book depth.

With a buy limit of $0.51, only the 20 contracts offered at that price are immediately available in this example. The remaining quantity may wait or be cancelled according to the order instructions. A limit avoids paying above the chosen price, but it cannot create sellers at that price.

An exit has the same problem in reverse: the relevant prices are the bids willing to buy your contracts. Do not value an immediately saleable position using the best ask. If you need to exit quickly, record the bids and their size as well as the purchase side.

For each venue, note the exact outcome, observation time, best bid, best ask, quantities at each price and the total intended position. Calculate a weighted purchase price and a separate executable exit value. Then add the applicable fees. The result is a comparison for that size and moment, not a permanent liquidity ranking.

Our liquidity and spreads guide explains the vocabulary. Robinhood’s order and liquidity explainer provides a platform-specific reference. We have not collected a live cross-venue order-book sample for this article.