Polymarket arbitrage: fees, order book and live spreads

Every market is a pair of YES and NO shares that trade between 1¢ and 99¢. When the market resolves, the winning share pays $1 and the losing one pays nothing. Prices come from a central limit order book, so what you pay is the best ask plus whatever size you need beyond it.

Multi-outcome events (an election, a championship) are groups of separate YES/NO markets, one per outcome. That matters for matching: a Polymarket outcome such as "Inter Milan" inside "Serie A Champion" has to be paired with the same outcome on the other venue, not with the event as a whole.

Polymarket charges taker fees on a growing share of markets. The fee depends on the market's category rate and on price: it is highest near 50¢ and shrinks toward the extremes. Geopolitics markets have stayed fee-free. Makers pay nothing.

ArbSonar reads each market's own fee schedule from the Polymarket API and applies it to the leg at the quoted price, so the cost you see already includes the Polymarket fee. Rates have changed several times in 2026, so check the order ticket before you trade.

Polymarket lists more markets than any other venue we scan, and smaller venues often copy its events. Predict.fun even links many of its markets to the Polymarket market they mirror, which lets us pair them by ID instead of by wording. The result: the majority of live spreads have one leg here.

The flip side is depth. A popular Polymarket market can hold thousands of dollars at the best ask, while the matching market on a smaller venue holds a few hundred. Your trade size is limited by the thinner side, and the scanner caps profit at that size.