Depending on the size of the area that you need to fence in, fencing costs can be substantial. People who own cattle or other livestock and need to fence in hundreds of acres spend an enormous amount on fence building and maintenance. For smaller operations, cheaper alternatives are available.
An electric fence requires you to pay for the electricity to run it, but its materials are cheaper and easier to install than a heavy wooden fence. An electric fence requires only posts, a lot of wire and a bit of electrical equipment. If you have a huge area to fence in and not much upfront capital to pay for it, an electric fence may suit your needs.
Occasionally you can find people who are removing fences and just want to get rid of them. Fences acquired in this way aren't always in great shape, but if you're moving them anyway you can get rid of the parts that are rotten or damaged and use the good parts. Keep an eye on online trading sites, or put up a message online or your own wanted sign in your town and try to locate fences that people want removed.
An effective, though not necessarily attractive, fence can be made out of shipping pallets. Stack them in a zigzag line and screw, nail or lash their sides together to make a fairly strong fence. Pallets are used by the millions every year, and many companies will be happy to give them to you. If you are looking for the cheapest option, and aren't afraid of a bit of hard work moving hundreds of pallets around, this option can't be beat.
Many fences have traditionally been made out of living plants. This is not a quick fix, as it requires several years for the plants to get large enough to be effective. Dense and thorny plants that are hardy and local to your area are best, as they will be likely to survive and can be obtained more cheaply than exotics. If you have a large property, try to find stands of small plants that are appropriate for fences. You may be able to uproot them and replant them in rows where you want the fences to be. This method requires some serious planning, but can result in a self-regenerating, zero maintenance fence.