How much do Kalshi and Polymarket fees actually cost?
Most of a cross-venue price gap is fees, and the cost is not flat. It peaks on contracts trading near 50¢ and falls away at both extremes, because Kalshi's fee scales with price × (1 − price). On the pairs currently on the board, fees cost 2.48pp of edge in the worst band and 1.10pp in the best.
Cost by price band
| Contract price | n | Average fee per contract | Edge given up |
|---|---|---|---|
| under 20¢ | 4 | 0.75¢ | 1.14pp |
| 20–40¢ | 3 | 1.54¢ | 2.21pp |
| 40–60¢ | 4 | 1.31¢ | 2.07pp |
| 60–80¢ | 3 | 0.81¢ | 2.48pp |
| over 80¢ | 4 | 0.61¢ | 1.10pp |
Small samples — read the n column before drawing a conclusion from any single band. The shape, though, is structural rather than statistical: a fee proportional to price × (1 − price) is largest exactly where the contract is most uncertain.
By venue
| Venue | n | Average fee per contract | Edge given up |
|---|---|---|---|
| kalshi | 9 | 1.28¢ | 1.84pp |
| polymarket | 9 | 0.69¢ | 1.63pp |
Kalshi charges an explicit trading fee. Polymarket's cost is the spread you cross plus gas, which is why its per-contract number can look smaller while giving up more edge on the same pair.
Why this decides most comparisons
A 2% gap between venues is a headline. After a fee of about a cent on each side it is not a trade. That is the single most common reason a spread that looks free is not, and it is why every number on the arbitrage page is shown twice — raw and net.
Fees change. These are measured from live pairs at the time this page was served, not copied from a schedule that may be out of date.