What it charges

Reported fees divided by reported contracts, on days carrying both. Days without a defensible numerator are absent, not drawn as zero.

Why a fee rate moves at all. Most of these venues charge a parabola in price — roughly Θ × contracts × p × (1−p) — which peaks at 50¢ and falls to nearly nothing in the tails. So an effective rate tracks where on the probability axis the venue traded, not a change in its published schedule. Polymarket's schedule did change, on 17 September 2026 (the coefficient) and 25 September 2026 (a surcharge on parlays).