All-in cost

Also called breakeven price · cost basis

Price plus fees per contract — your true breakeven, and the only number position sizing should ever use.

The most common sizing error in these markets is computing edge off the sticker price. In this codebase the cost of a contract is price plus the venue's fee at that price, and every downstream number is derived from it: your edge is your probability minus that cost, your net payout multiple is one minus cost over cost, and your breakeven probability is the cost itself.

Worked on Kalshi. A contract asks 40¢ and you honestly believe the outcome is 50 percent. The fee at that price is about 1.7¢, so the all-in cost is 41.7¢ and the edge is 8.3¢ per contract. Full Kelly on those numbers lands near 14.2 percent of bankroll. Run the identical trade with no fee and it computes to 16.7 percent. Two and a half points of optimal size evaporated into cost before the market moved at all — and that is on a trade that still works.

When cost exceeds your probability, the correct position is zero rather than small. The sizing function returns zeros in that case by design, because a fee-aware calculator that still suggests a token stake on a negative-edge trade is worse than no calculator.

Where this shows up

Related terms

Definitions describe how Polymarket and Kalshi behave and how WhaleTracks models them; venue rules and fee schedules can change, so confirm anything you size a position on with the venue itself. WhaleTracks is informational analytics, not financial advice. Past performance does not guarantee future results.