Sportsbook vs exchange price comparison: what the same outcome costs in odds and in cents

Free tool · 18+ · Not betting advice · By BetG8

A sportsbook quotes -110. Kalshi quotes 50 cents. Polymarket quotes 50 cents. Those are three prices for the same thing, a dollar paid if the outcome happens, and none of them is the number you actually pay. The sportsbook's price hides its margin inside the odds; the exchange's price leaves its fee off the ticket. This tool puts them on one footing: enter both sides of the book's line and the exchange price for one side, and it returns the book's vig-free probability, the exchange's all-in cost after the taker fee, and which one sells the outcome for less.

How it works

Every outcome has a true price: the probability that it happens, expressed in cents on the dollar. A sportsbook sells you that outcome at a mark-up, and the mark-up is invisible because it is spread across both sides of the line. At -110 each way, each side implies 52.4 percent and the pair adds to 104.8 percent. The 4.8 points over 100 is the overround; as a share of what is wagered it is about 4.5 percent, which is the book's expected hold on a balanced market. Remove it by dividing each side by the total and you get the book's own fair estimate, 50 cents a side. So the sportsbook is selling a 50-cent outcome for 52.4 cents.

An exchange sells you the same outcome at whatever another trader will take, plus a fee. On Kalshi the fee for an order that fills immediately is 0.07 × P × (1 − P) per contract, which peaks at 1.75 cents when P is 50 cents and falls toward both ends; a 51-cent contract costs 51 + 1.75 = 52.75 cents all in. On Polymarket the formula has the same shape with a smaller coefficient in most categories: 0.05 for sports gives 1.25 cents at 50 cents, 0.04 for politics gives 1 cent. The tool computes the fee on the price you enter, adds it, and compares the result with the sportsbook's price for the same side. It also shows the exchange's all-in cost against the book's vig-free number, which is the closest thing to a neutral benchmark either venue publishes.

The table assumes the exchange quotes exactly the book's fair price, which it rarely does, and it uses the book's two-way line to define fair, which is generous to the book. Even so, the pattern holds across the range: a taker on either exchange pays less than the sportsbook's vig on a balanced line, and the gap widens on favourites, where the book's hold is spread over a bigger price and the exchange fee shrinks with P × (1 − P). The exception is the longshot end, where Kalshi's fee as a share of stake is highest and a book with a small overround can come close. We walked through the two business models in sportsbook versus prediction market.

Reading the result

The tool returns four numbers worth keeping. The sportsbook's implied probability is what you pay per dollar of payout; its fair probability is what the book thinks the outcome is worth once the margin is stripped out. The exchange's all-in cost is the cent price plus the fee on that price, plus any spread you tell it you will cross. The break-even probability is the same figure read the other way: the outcome has to happen at least that often for the position to return your money over many repetitions. When the exchange all-in cost is below the sportsbook's implied probability, the exchange is the cheaper seller of that outcome, and the difference, in cents per dollar, is what the comparison is worth. Multiply by the stake and it becomes dollars.

Two things the number cannot tell you. It cannot say whether either venue is right about the probability; it compares prices, not forecasts. And it cannot see the exchange's order book depth. A 51-cent quote that holds for 20 contracts and jumps to 54 cents for the next 200 is not a 51-cent price for a $200 order, which is why the spread field exists and why we measured real spreads across venues in what the spread really costs.

Limits

The multiplicative de-vig method assumes the book's margin is spread in proportion to each side's price. Books often load more margin on the longshot, so the fair price on a +300 side is probably a little lower than the tool shows and the fair price on the -400 side a little higher. Kalshi's fee is rounded up to the next cent on the whole order, so a one-contract trade pays at least 1 cent whatever the formula says, and some Kalshi series carry a multiplier other than 1; the schedule at kalshi.com/fee-schedule governs. Polymarket's rates are those published in its fee documentation as of October 2026 and are subject to change; its geopolitics markets are fee-free and its makers pay nothing on every category. Neither fee figure includes deposit costs, which on Kalshi can be up to 2 percent by debit card and on Polymarket depend on how you convert to USDC. The tool does not check whether either venue is available where you are; sports contracts are geofenced or under court order in several states, and the current position is in our 50-state tracker. For Kalshi against Polymarket on the fee alone, see Kalshi versus Polymarket fees; for converting odds formats without the comparison, use the odds converter.

FAQ

How do you compare a sportsbook line with a Kalshi or Polymarket price?

Convert both sides of the line to implied probability, divide each by the sum to remove the vig, then compare that fair cent figure with the exchange price plus the taker fee on that price. The lower all-in number is the cheaper way to buy the outcome.

What does a -110 line cost after the vig is removed?

-110 implies 52.4 percent. With -110 on both sides the fair probability is 50 percent and the book's hold is about 4.5 percent of the money wagered.

How much does Kalshi charge on a 50-cent contract?

1.75 cents per contract for a taker under the 7 July 2026 schedule, so 51.75 cents all in. A maker on a listed series pays 0.44 cents.

Are Polymarket fees the same in every category?

No. The published rate is 0.07 for crypto, 0.05 for sports and most other categories, 0.04 for politics, finance and tech, and zero for geopolitics. Makers pay nothing.

18+. This tool performs arithmetic on numbers you enter. It is not a recommendation to bet or trade on any venue, and it does not know whether a venue is open to you where you live.

Related

• Sportsbook vs prediction market: which one actually gives you a better price?
• Kalshi vs Polymarket fees compared
• Kalshi fees explained: taker, maker and the 0.07 formula
• What the spread really costs on prediction markets
• Is Kalshi legal in your state? The 50-state tracker
• Odds converter

Sources

• Kalshi, Fee schedule, effective 7 July 2026, for the 0.07 taker and 0.0175 maker formulas, rounding and series multipliers.
• Polymarket, Fees, for the rate × price × (1 − price) formula and the category rates, read October 2026.
• BetG8, Kalshi vs Polymarket fees compared and what the spread really costs.

← All stories