Are you on a meter or a well? If you have a meter, be sure to check that it's a modern, newer designed meter that doesn't allow water to go both ways through the meter. I had a $600 water bill for an empty house that I was flipping. The water utility said that there is no way that the meter was wrong, but when I told them about water surging from their pumps that causes water pressure to rise and fall, allowing water to go through the meter, then back out again, which keeps turning the meter, but no water is used, they replaced the meter that week and eventually forgave the bill. They never admitted that I was right, and the guy in charge kept telling me that it was impossible, but they sure did change that meter real fast!!! I was given the information about surging water pressure causing meters to read false usage amounts from the guy in charge of my water utility, so my source was solid.
On long runs, glued together PVC can crack when the ground freezes. When it freezes and there is enough moisture in the ground, it's just like an ice cube in a tray. This is what causes most foundation failures too. The crack can be so small that you never see the water at the surface. It can also follow the length of the pipe until it finds a gopher hole, or something like that, and continue to run without ever being seen on the surface. Your only clue besides the big bill for water usage is an area that is green, where everything else is dead looking.
If your water pressure is too high, usually above 60 pounds, water will continue to flow through the valves in the toilets. You might never know this is happening since it's such a slow process, but it adds up to serious money by the end of the month. Especially if you have several toilets.
If you have to replace your water line to the house, spend the extra money to get gasketed water lines. The pipes slide into each other and allow for movement in the ground. If you ever see any water utility lines being installed, all of the pipes will have gaskets at one of their ends. This is what is used in every new water and sewer line installed around my area of the country, and I would guess, it's done the same everywhere else. Just deeper in colder climates.
(OK, granted, many of the houses were built before indoor plumbing, but still.)