A betting exchange is a different business from a bookmaker

A bookmaker is the counterparty to your bet and profits from the margin built into its prices. An exchange is a venue: it matches your bet against another customer's opposite bet and charges a commission on winnings. Nobody at an exchange has a position in the outcome, which changes almost everything about how the prices behave.

Because the exchange does not carry risk, it has no reason to shade a price to attract one side, and no reason to restrict a customer for winning. Its revenue is a function of volume rather than of margin, so its incentive is for the market to be as liquid and as accurate as possible — which is why exchange prices are frequently used as a reference for what a fair price looks like.

It does not mean an exchange is always cheaper. Commission on a heavily backed favourite can exceed a bookmaker's margin, and a thin market can have no meaningful money on the other side at any price worth taking.

That is also the expensive part. The advertised exchange price is quoted before commission and the bookmaker's is not, so a naive comparison flatters the exchange on every selection. Doing it properly means subtracting commission from the exchange's net return every time.

What to do with it. Compare net returns, not prices. Work out what 100 actually pays back at each venue and pick the larger number.