Effective fee at fifty cents

This is our own term, not the industry's, and that is exactly why it needs a page. The quantity sits on every card in this catalogue that carries a trading fee, and no venue's documentation contains the phrase or the number. The cards themselves say it longhand — "1.25% of a dollar of payout on a fill at 50 cents" — because until this page existed there was nowhere to send a reader who wanted to know what the fee column meant.

How it works

A contract here pays $1.00 if it resolves your way and $0.00 if it does not, and it trades between 1 and 99 cents. That gives a fee four different things to be charged on, and all four are in production use in this sector:

• A coefficient on expected earnings. Kalshi's schedule defines the base as the maximum earnings on a contract multiplied by the implied probability of making them, which is P x (1 - P), and applies a coefficient to it. Polymarket, Polymarket US and Futuur use the same shape with different coefficients.
• A percentage of what the fill costs. Limitless and Myriad publish percentages of the money that changes hands, which makes their headline numbers look large.
• A flat amount per contract. ForecastEx charges $0.01 per contract to each side of every execution, independent of resolution.
• Nothing on the fill at all. PredictIt charges on profit and on withdrawal instead.

The field takes the cost of one contract, bought at 50 cents, taking liquidity, expressed as a percentage of the $1.00 a winning contract pays. In that unit the four bases become one column:

Read the first and the fifth rows together. Limitless publishes a number nearly twice the size of Kalshi's and charges about fourteen percent less for the same trade.

Why the midpoint, and not an average

50 cents is where the coefficient formula peaks, it is where the symmetric curve is widest, and it is the one price at which every one of the four bases produces a number. A volume-weighted average would be defensible arithmetic and undefendable practice: no venue publishes the price distribution it would need, and the figure would move every month without a single fee schedule changing.

The two limitations, stated rather than hidden

On a coefficient venue the number is the peak of a curve, so a reader who mostly trades long shots pays roughly a third of it. On ForecastEx, and on Robinhood inside the range where its cent cap binds, it is a plateau — 1% at 50 cents is also 1% at 5 cents. Ranking a peak against a plateau is correct at the midpoint and can invert anywhere else: at 5 cents, ForecastEx's flat cent costs about three times what Kalshi's coefficient does on the same hundred contracts.

Why it sits next to the data APIs

Because it is a normalisation rule rather than a property of any venue. Nobody charges it; the catalogue computes it so that a fee can be sorted, filtered and read across sixty cards, which is the same job the prediction market data APIs section does with prices. What a trade actually costs is filed there for the same reason, and it is the working that this definition is the conclusion of.

Why it matters here

You will go looking for 1.75% in Kalshi's fee schedule and not find it. It is not in there. The schedule says 0.07, and the card says what 0.07 comes to on one contract at the midpoint. Each card's Pricing section carries the published formula, the coefficient and any per-category table, because one number cannot hold a schedule with eleven rows.

You will double it by accident. On 100 contracts at 50 cents you hand over $50 and pay $1.75. That is 3.5% of your stake and 1.75% of the $100 those contracts pay. Both are arithmetically correct, they differ by a factor of two, and only one of them can be compared with ForecastEx's flat cent.

You will halve a venue by accident in the other direction. Limitless's 3.00% and Myriad's 1% are percentages of the fill. Writing 3.00 into a card's fee field states a charge twice the real one. The tell is an extreme price — a rate that would charge three cents on a contract costing one cent is a rate on the stake, not on the payout.

You will apply it where nothing is charged per fill. A parimutuel pool takes its cut once, out of the pool, at settlement, and a play-money venue takes nothing at all. Neither has a taker to charge. The field is then null, which renders as "None" and is a claim that nothing is taken — never a number, and never absent, which is the separate state reserved for a charge that exists and could not be priced.

You will sort the listing and trust the order. It is the right order for the trade the number describes and not necessarily for yours. If you mostly buy at 10 or 90 cents, rank the coefficient venues against each other and read the flat-fee ones off the second table in the fee guide instead.