Prediction market fees and rebates, calculated

Yes. Polymarket was fee-free for years and rolled fees in through 2026. Takers pay shares x rate x price x (1 - price), where the rate is set by category, from 0 (geopolitics, which is free) up to 0.07 (crypto). Makers pay nothing and earn a rebate.

Kalshi charges takers round_up(0.07 x contracts x price x (1 - price)), rounded up to the next cent on the whole order, so 100 contracts is not simply 100 times the one-contract fee. S&P 500 and Nasdaq-100 markets use a lower 0.035. Makers pay nothing on most markets.

It depends on the category. Polymarket politics and finance markets (rate 0.04) undercut a Kalshi general market (0.07), Polymarket crypto (0.07) matches it, and Kalshi's index markets (0.035) beat both. On a 100-share trade at 50¢, Polymarket sports costs $1.25 against Kalshi's $1.75.

On a 100-share trade at 50¢, where the fee is at its worst: Opinion is about $0.50, predict.fun $1.00, Polymarket $1.25 on sports (0 to $1.75 by category), Limitless $1.50, Kalshi and Gemini $1.75, rain.trade $2.50, and Robinhood $3.50. Kalshi's index markets are the cheapest at $0.88. On most venues, quoting as a maker takes the fee to zero.

A maker posts a resting limit order instead of crossing the spread, and pays no fee. On Polymarket you also earn a rebate, roughly 15% to 25% of that category's taker fees, shared across the resting liquidity, so the exact amount depends on your slice of the book. Kalshi, predict.fun, Opinion and Limitless charge makers nothing but pay no rebate; Gemini actually charges makers a reduced fee; and Robinhood charges its commission on every order.

Robinhood routes to Kalshi but adds its own commission: a 0.10 coefficient against Kalshi's 0.07, plus Kalshi's $0.01 per-contract exchange fee, on every order with no maker break. That works out to $3.50 on a 100-share trade at 50¢, the priciest here. A Robinhood Gold subscription halves the commission coefficient to 0.05.

Your effective price is the nominal price plus the fee spread across your shares, and it is the number that actually matters. Buy at 60¢ in a Polymarket sports market and you are really paying about 61.2¢, which is also the win probability you need just to break even. Quote as a maker and your effective price drops below nominal.

Trading fees are separate from moving money. ACH bank transfers are free on almost every venue, but card deposits cost 2% to 3.5% (Kalshi 2%, Gemini 3.49%) and Gemini charges $25 on a US wire withdrawal. The on-chain venues (Polymarket, predict.fun, Limitless, Opinion) sponsor gas, so trades are gasless through their managed wallet, though a self-custody wallet pays gas and bridging funds onto the chain costs extra. Robinhood adds an optional $5-a-month Gold plan that halves its commission.

A few levers. Quote as a maker rather than crossing the spread: the fee goes to zero, and on Polymarket you earn a rebate. Avoid trading right at 50¢, where the parabolic fee peaks; it shrinks toward 0 and 100¢. Pick a cheaper category or venue, such as Polymarket politics or finance, Kalshi's index markets, or Opinion. On Robinhood, Gold halves the commission. And remember the spread usually costs more than the fee, so a patient limit order beats a market order.