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Analysis from MatchedBettingBlog suggests that margins on extra-places markets can reach as high as 50 per cent. Bookmakers open with a cautious book and adjust as money comes in. The first show of prices in the morning — often called the early prices — tends to carry a wider margin than the Starting Price. On the raceday market itself, there are consistent patterns. Ante-post markets on the Cheltenham Gold Cup or Champion Hurdle typically run between 140 and 160 per cent, which is lower than the National but still significantly wider than raceday. The aggregate effect across 20 runners adds up to a much wider margin than in a six-runner race.

On horses that are supported in the market — money coming in, price shortening — BOG adds nothing, because the SP is lower than the price you took. Check the exchange price on your selection before placing with a bookmaker. Flutter Entertainment, which owns Betfair, reported global revenue of $14 billion in 2024 across 13.9 million average monthly users, but the exchange is only one part of that portfolio, and liquidity varies enormously by market. On a Monday afternoon novice hurdle at a minor track, liquidity is thin. On feature races — the Champion Hurdle, the Gold Cup, the Grand National, Saturday afternoon handicaps at major tracks — the exchange market is deep enough that you can back at the displayed price without difficulty. Over time, backing at exchange odds rather than bookmaker odds produces a measurably higher return on investment, assuming you can find liquidity on your chosen market.

The days when betting shops competed fiercely on price have given way to an era where online platforms sometimes offer “good enough” pricing, knowing many customers will not check elsewhere. Before placing multiple bets, check the specific bookmaker’s terms—assuming BOG applies universally is a common and costly mistake. BOG is a tool for capturing extra edge when your original selection was sound; it does not substitute for sound selection.

Value exists in the gap between your probability assessment and the bookmaker’s implied probability. A horse at 2/1 (33% implied probability) that you assess as a 25% chance is not—even though the shorter price might feel safer. A horse at 4/1 (20% implied probability) that you assess as a 25% chance is a value bet. The more fundamental question is whether those odds represent value—whether the implied probability understates the horse’s true chance of winning. Mobile apps from comparison sites make price-checking quick even when betting on the move.

Strategic use of BOG requires understanding when to bet and when to wait. This promotion exists because bookmakers are competing for early-morning turnover. A six-runner race where implied probabilities sum to 112% has a 12% overround—meaning the bookmaker’s theoretical margin is 12% before any punter places a bet. But without calculating implied probability, you cannot begin to answer that question systematically.

Using the exchange as a benchmark — even if you ultimately bet with a bookmaker — is one of the most effective habits a punter can develop. If the exchange offers 7.0 and the bookmaker offers 5/1, the bookmaker is taking a significantly larger margin on that runner, and you should either back on the exchange or find a bookmaker whose price is closer to the exchange level. If the exchange offers 6.0 and the bookmaker offers 5/1, the bookmaker is competitive after commission. The practical approach for most punters is to use both.

On a £10 each-way bet, that difference is £5—pure margin handed to you when bookmakers compete on place terms. At 1/5 odds, your 10/1 shot returns just 2/1 (plus stake) on the place portion. These are standard industry terms, but bookmakers frequently enhance them for major football betting sites not on gamstop races or as ongoing promotions. If prices consistently drift after you bet, something in your analysis may need adjustment.

Where a bookmaker might offer 5/1 on a horse with a 120 per cent book, the exchange might show 5.5 (or 9/2 in fractional terms) on the same runner with a total market overround of 103 per cent. The overround on the Grand National ante-post market can reach as high as 180 per cent — a level that would be considered predatory in any raceday market. At the upper end, you find big-field handicaps with 20 or more runners, where each horse price can be inflated by a few ticks without any individual punter noticing the difference. If you bet at a bookmaker with a consistently higher overround, you need a higher strike rate just to break even. Checking the overround before you bet is not something most punters do, but it takes less than a minute with a calculator or an odds comparison site. The worst overrounds appear in ante-post markets, where the uncertainty justifies — in the bookmaker’s view — margins that can reach extraordinary levels.

The overround is 5.9 per cent, meaning the bookmaker has built a theoretical edge of just under 6 per cent into the market. It is the bookmaker’s theoretical profit margin before a single bet is settled. In practice, they always sum to more — typically between 110 and 130 per cent for horse racing — and that excess is the overround. In a perfectly fair market with no margin, the implied probabilities of all runners in a race would sum to exactly 100 per cent. Understanding those differences is where value lives. The bookmaker margin is built into every price, but how wide that margin is varies enormously between operators, between race types, and between the fixed-odds market and the betting exchange.