The payoff contract
Enter the average gross winning trade, average gross losing trade and the same round-trip cost per trade. Expected payoff is win probability times the gross win minus loss probability times the gross loss minus cost.
The breakeven expression
The required win probability is gross loss plus cost, divided by gross win plus gross loss. If cost exceeds the gross win, no win rate at or below 100% breaks even under these assumptions. Real outcomes and costs are distributed, not fixed.