when a bookmaker prices an event they price it to what we call the 115% market (110% on the internet because they have fewer overheads). this means that no matter who wins they will pay out 100% and keep the 15%.
for instance in a world cup final (outright betting is used for the example) there are to possable outcomes, home win and away win. if they were evenly matched the prices should be even money on each team (1/1). but they wont be, they'll be priced at 5/6 each.
5/6 is given on the condition that both teams are perfectly matched, that way the bookmaker will take half there bets on home team and half on the away team, they'll pay out at 5/6 on half there bets so for every £100 that is staked on each team they'll pay out £183.33 giving them £16.67 profit, roughly 15%.
of course, no world cup finals are totaly evenly matched which is why you get the variation in pricing, ie:
11/8 home team and 2/1 the away team, roughly speaking.
this pricing shown above takes into acount the home teams advantage against the away team, say england vs USA, england or more likely to win. Obviously, england have a higher chance of wining against USA than the other way around (no offence intended for our foreign cousins), because they have a higher chance of winning they will take more money on england, hence the lower odds, USA are effectivly the long shot of the two teams, hence the larger price.
If however the bookmaker takes a huge amount of cash on USA the will adjust the priceing, say 11/10 england and 6/4 USA. this will correct the original price error and will encorage people to bet on the home team (england), therefore correcting the "book". this is an extreme scinario however, because normaly the bookmaker will restrict betting on the outsider when they get a large influx of, what is really unusual betting.
Hope this helps.
I run a betting shop,also 15% may sound like a lot but once we've paid our overheads it really isn't!