Kalshi and Polymarket combined for 1.33 million of the 2.8 million NFL betting app downloads recorded in Week 2, good for roughly 47.5% of the category — up from 19% growth through the season's first two weeks, per Sensor Tower data cited by Citizens (via pressbee.net). Polymarket alone leads all operators at 1.37 million downloads through two weeks, with Kalshi close behind at 1.31 million, while DraftKings sits third at 805,000 and FanDuel fourth at 502,000 (pressbee.net). That's the headline. The more useful number is buried three paragraphs down: Kalshi's average implied vig across 30 pricing observations collected September 18 was 4.22%, versus 4.43% at FanDuel and 4.50% at DraftKings, excluding transaction fees (pressbee.net).
That gap is real but it's not where the money is. On combo bets — favorite-and-over parlays — Kalshi's implied vig jumped to 26.4% against 23.9% at DraftKings, running roughly 6% higher than FanDuel's pricing before fees (pressbee.net). Combos still drove $26 billion of Kalshi's trailing-30-day volume, 53% of its total book, up from a 44% average over the prior 90 days (pressbee.net). The exchange is winning on straight moneylines and totals and losing badly on the parlay product that's carrying more than half its volume. If you're pricing exchanges against books, the edge only shows up on single-outcome markets — anything stacked into a combo flips the advantage back to DraftKings and FanDuel.
Where the Real Split Is Sitting This Week
The cleanest cross-market disagreement on the board isn't an exchange-versus-book story at all — it's a bets-versus-money story inside a single sportsbook, and it's happening on the Ravens-Cowboys game in Rio, the NFL's first regular-season contest there and one of Week 3's marquee matchups (DraftKings). Oddsmakers opened Baltimore as a 3-point favorite, and the line moved to 3.5 by midweek despite the Cowboys drawing more bets (BetMGM). As of Friday morning, Dallas held 59% of spread bets and a striking 92% of the money wagered on the moneyline to win outright — yet BetMGM's latest data has flipped, with 58% of moneyline bets now backing the Ravens as favorites (BetMGM). Spread bets still lean Cowboys, with 52% of Saturday-afternoon action on Dallas covering 3.5, even as the moneyline sentiment reversed (BetMGM).
That's a market talking out of both sides of its mouth inside the same book, and it's exactly the kind of seam this column looks for — not because Polymarket and DraftKings disagree, but because bettors are pricing the spread and the moneyline on the same game as if they're different propositions. Some of that is noise from injury news: Baltimore ruled out offensive tackle Ronnie Stanley and lists Zay Flowers as questionable, which explains hedge activity on the total more than the spread (BetMGM). The over/under has moved from 52 to 53.5 points with 77% of money on the over as of Friday, consistent with how these Brazil games have trended in the past two seasons (BetMGM).
The Structural Read
Put the two data points together and the picture is less about which platform is "right" and more about where liquidity concentrates. Kalshi and Polymarket are winning the download war and pricing single-outcome bets slightly tighter than the traditional books, but the combo market — where most of Kalshi's actual volume lives — runs materially worse for the bettor than FanDuel or DraftKings (pressbee.net). Meanwhile a straightforward NFL sportsbook line can show a moneyline and a spread pulling in opposite directions on the same 60 minutes of football, purely because two different bet types attract two different customer bases (BetMGM).
Neither gap is an arbitrage in the strict sense — you can't middle a moneyline against its own spread without accepting real variance. But it's the same signal I keep coming back to: efficiency in these markets is conditional on bet structure, not platform brand. Watch whether Kalshi's combo vig compresses as its share of NFL volume matures past this season's download surge, and watch whether Sunday's Ravens-Cowboys result settles the bets-versus-money split in Baltimore's favor before the next international window in London later this fall.
