The Price of a Bet: Measured Bookmaker Margins Across 52 Bookmakers, 22 Competitions and Player Props

The bookmaker's margin (overround) is the price of a fixed-odds bet, yet systematic public measurements of it are scarce, and none existed for player-proposition markets. Using 4.27 million odds observations recorded between 29 June and 2 August 2026, this paper measures margins on 2.7 million cleaned markets across 52 online bookmakers, 22 competitions and three market types. Four regularities emerge. First, a market-type price ladder: two-way head-to-head margins average 4.77%, spreads 4.90%, totals 5.45%, three-way match betting 7.67%, and player props 7.93% — to the author's knowledge the first public measurement of prop-market margins. Second, margins carry a term structure, peaking one to three days before the event (5.23%) and compressing to 4.71% in the final six hours. Third, licensing regions price differently: Australian-licensed operators average 5.25% against 4.72% for US-licensed and 4.55% for offshore/international books. Fourth, operator dispersion is large — mean two-way margins range from 3.16% to 5.78% across books, an 83% difference in the cost of identical bets. Companion study and open data: https://betbetter.world/studies/bookmaker-margins