The short version
When DraftKings posts −110 on both sides of a game, that's not their estimate of fair probability. It's their estimate plus a markup. The implied probability of −110 is 52.38%. If both teams pay −110, the two probabilities add to 104.76%. That extra 4.76% is the bookmaker's fee, called the overround or vig. Strip it out and you get the book's actual fair line, both teams at 50/50.
Polymarket doesn't work that way. When you see Thunder championship priced at 0.524, that's the literal price of the YES token. By construction, YES + NO add to 1.000. There is no hidden markup baked into the displayed price. The mechanism is fundamentally different.
Sportsbooks are bookmakers. Exchanges run an order book.
A retail sportsbook is the counterparty on every bet you take. They post lines they think will balance their action and they've baked in a margin so they win on average even if the line is right at fair. That margin is the vig. The price you see already includes it.
Polymarket, Kalshi, and Betfair Exchange are order-book exchanges. The price you see is the lowest ask (or highest bid) some other trader has resting on the book. They're willing to take the other side at that probability because they think it's right. When you fill against them, the platform takes a small fee, but only on the fill, not embedded in the price.
Same outcome, two different price structures:
Sportsbook (DraftKings):
Outcome A: -110 → implied 52.38%
Outcome B: -110 → implied 52.38%
Sum: 104.76% (4.76% vig)
Fair after devig: 50.00% / 50.00%
Order-book exchange (Polymarket):
YES token: 0.500
NO token: 0.500
Sum: 1.000 (no vig in the price)
Fee: ~2% taker on fill (separate from price)What this means for fair value
When you compute "fair" from sportsbook odds, you have to devig first. Multiplicative devig is the simplest. Power-method or worst-case-shin are more accurate. The CLV.gg sharp consensus calculator per-book devigs each contributing book's YES + NO implieds, then weighted-averages across the sharp set. That weighted average is your sharp fair.
When you read a price off Polymarket, you don't devig it. It is already a probability. The mid-price of an order book is what the next marginal trader believes the fair value is, after you back out the spread. Treating the Polymarket price like a sportsbook line by devigging it would be mathematically wrong because there is no overround to strip.
The fee structure isn't free
Polymarket having no vig in the price doesn't mean trading there is free. Every taker fill pays a fee, typically around 2% on US-eligible markets and sometimes higher. The catch: the fee comes off your fill, not the price. So when you compute edge as (fair − poly) / poly, that's the gross edge. Subtract your taker fee to get the net edge that clears.
Maker orders (limit orders that rest and get filled) typically pay a much smaller fee or zero. If you can be the resting bid, you don't need as much edge to be profitable. Most users will be takers most of the time, so default your math to the taker assumption.
Why sharps run both venues
Two reasons. First, the order book can't limit you. It doesn't know who you are, it just matches the next marginal buyer with the next marginal seller at the agreed price. Skilled bettors who'd been banned, capped, or "risk-managed" off retail sportsbooks suddenly had a venue that wanted their action. Same trade, no review.
Second, price discovery on Polymarket and Kalshi has gotten good on high-volume markets. The election markets routinely outperformed both polls and prediction-aggregator forecasts in 2024. The NBA championship futures clear hundreds of thousands per day. Real liquidity, real flow, and the prices reflect what someone is willing to fill, not what a bookmaker wants to advertise.
Sharps still run sportsbooks too. CLV.gg watches 15+ US sportsbooks for the same kind of mispricings, just on a different surface. The soft books that haven't limited you are where the +EV detector spends most of its time.
The takeaway
Don't apply sportsbook math to exchange prices. The price you see on a Polymarket token is a probability. The fair value comes from the sharp sportsbook consensus (devigged and weighted), not from devigging Polymarket. The edge formula is (fair − poly) / poly, fee-adjusted for the taker side. Different mechanism, different math, same goal: find the mispricings, then act on them.
Tools that do this for you
The CLV.gg sharp consensus calculator and edge calculator ship the same math the live engine uses. Both are free, no login. The full live edge feed and historical CLV tracking is in the app, see pricing.