The cost of a bet isn’t just the amount you stake – the margin there’s added to the odds. Bookmakers can add whatever margin they want and this makes for odds you bet with different to the true odds of something happening. Different bookmakers will have different margins which won’t be displayed anywhere. So you’ll have to work them out. The one thing to remember is that the higher the margin the less value there is for the better. In addition to adding in a margin for the odds, a bookmaker also balances the book to guarantee a profit they will take that and order the potential outcomes for an event then change the odds based on how much they stand to lose if they have to pay out for any particular outcome.
Changing the odds will make one option more appealing than the other and over time the bookmaker will find a balance to ensure their profits outweigh their losses. Because a bookmaker adds a margin and can balance the book. They will normally collect more than they have to pay out. It doesn’t always work out like that but it works out enough to make money and run a business. You can’t control how the bookmaker makes money but you can maximize your own profit.