Function compute_commission 

pub fn compute_commission(
    fee_rate: Decimal,
    size: Decimal,
    price: Decimal,
    liquidity_side: LiquiditySide,
) -> f64

Computes a pUSD commission using Polymarket’s fee formula.

fee = C * feeRate * p * (1 - p) where C is shares, feeRate is the effective taker rate from the market’s feeSchedule, and p is the share price. Fees peak at p = 0.50 and decrease symmetrically toward the extremes. Only taker fills pay; maker fills always return zero. Rounded to 5 decimal places (0.00001 pUSD minimum).

The fee_rate here is the effective rate from feeSchedule.rate (e.g. 0.03 for 3%), not the fee_rate_bps field on a V2 trade response. The response field is the post-trade rate that actually applied; under V2 the fee is no longer carried in the signed order, so we compute commissions from the instrument’s fee schedule rather than reading any cap off the order body.

References

https://docs.polymarket.com/trading/fees