One cent of edge can disappear in the fee line

An apparent spread is gross. The fee line can consume it before the position ever reaches settlement.

Costs can include platform charges, the bid-ask spread, slippage, and the opportunity cost of locked capital. Use the current fee terms for the actual order rather than an old screenshot.

Start with complete cost

Write the gross edge, intended size, fee basis, spread, estimated slippage, and expected lockup. If a number is unknown, keep a range instead of hiding it in a single optimistic estimate.

Imagine a two-cent gross edge with 0.8 cents of fees and 0.7 cents of slippage. The remaining half-cent is only an illustration, before timing and settlement uncertainty.

Do not average away the tails

Small fills can look attractive while larger size crosses several price levels. A partial fill can also leave you with an exposure that no longer has the original edge.

Record every fill and canceled order. Calculate net results in the same units and keep unmatched inventory separate from completed trades.

Test the no-trade threshold

Set a minimum net edge and stress it with pessimistic fees and slippage. If the result does not survive that range, the correct action may be to skip the trade.

Do not use a backtest's closing price as proof that the fee line was affordable. Re-check the live terms before placing an order, because the cost is part of the strategy.

Further reading: Prices and order books · Resolution rules · Current fees.

Explore the markets: Open Polymarket (referral link).