A basic homemade sprinkler system requires the same types of pipes as any professionally installed sprinkler system. Fortunately, PVC pipes are low-cost items at most home improvement stores. For example, a 10-foot section of PVC pipe costs about $2 at most stores. If you have a yard that's 100 square feet--keeping in mind that you'll need enough pipe to circle the perimeter of the grass--that means the total cost of the pipe, in most cases, will be less than $10.
You don't have to buy fancy sprinklers to get the job done. In fact, the least-expensive sprinklers are 2-inch pop-up sprayers that cost about $1 each at most home improvement stores. These sprinklers contain plastic bodies and nozzles that accomplish the same task as more-expensive varieties. For a 100-square-foot yard, you can use sprinklers with 90-degree nozzles on the 4 corners, and 8 sprinklers with 180-degree nozzles along the middle of the pipes. The total cost of all the sprinklers should be less than $15 for small yards.
All sprinkler systems, whether homemade or professionally installed, require the use of a backflow prevention device. These devices prevent water from flowing from inside sprinkler pipes to the main line--water that enters sprinkler lines is considered by most municipalities to be contaminated, so it should never re-enter the main water line. While professional sprinkler systems ofter contain a pressure vacuum breaker, these devices cost about $100 each. Instead, you can use a simple check valve at the start of the sprinkler system. These valves allow water to flow in only one direction, and cost only a few dollars on average.
You'll need to purchase fittings to connect pipes together and the sprinklers to the supply lines. PVC fittings cost less than $1 each at most hardware stores, and can be found in an abundance of sizes and styles. For a simple square lawn, you'll need to purchase at least 4 90-degree pipe fittings, as well as about 5 couplers to connect pipes on straightaways. Pipe fittings are glued together using a special PVC cement that bonds the plastic together, so you'll also need a jar of cement at a cost of a few dollars.