Each platform has a different cost structure and audience.
Polymarket and Kalshi are exchanges with structurally low fees (~1%) — prices closer to fair value, set by traders bidding against each other.
Sportsbooks build in a 5-7% margin (the vig or overround) that always favors the house. They also adjust prices based on which side the public is betting, not just the underlying probability. The disagreement between these three pricing mechanisms is what creates arbitrage opportunities.

Closing line value (CLV) is the difference between the price you got and the price the market settled on at event start. If you bet at +120 (54.5% implied) and the closing line moved to +110 (52.4% implied), your CLV is +2.1pp. Long-term, sharps consistently beat the closing line by 1-3pp. CLV is a better predictor of long-term profitability than short-term ROI because individual outcomes are noisy.

OddsBridge walks each prediction market's order book to compute the actual fill price at $1k, $5k, and $25k stake sizes — not the headline mid-price. For sportsbooks, it cross-references typical pre-game stake limits per book per sport. A leg is paper if the fill slips more than 10% from the headline, or if the SB limit is below the required stake. Partial sits between 2-10% slippage. Executable means under 2% on every leg.

Three reasons. First, prices move quickly — by the time you place both legs, one may have changed. Second, sportsbooks impose stake limits and may void bets if they suspect arbing. Third, prediction-market order books have actual depth limits: betting $5,000 on a market with $1,200 of resting depth fills at a far worse price than the headline. OddsBridge surfaces all three risks via the verdict label and depth indicators — see the methodology page.