Choosing rocks native to your region is the least expensive way to go. Quarries are generally cheaper places to get your rocks and will have many different sizes. Sometimes mother nature will provide your anchor boulders in your own yard. You can reuse rocks found in other areas of landscaping, making your need to add rocks less. Another great supply of rocks is from builders. When they clear the land to build, they discard the large rocks, and many will be happy to give you rocks, since it saves them time and work in disposal.
Another way to keep your costs down in building a rock garden is to have your rock garden multi-task. If you have an area of your yard or garden which slopes, a rock garden is the perfect solution to reducing water run off. The rocks will slow the water and give it time to absorb into the ground, stopping it from flooding other parts of your yard or garden. Adding limestone rocks will aid in this, as they will absorb the water and generally have depressions in the rocks which allow for the planting of moss and other shallow-rooted flowers. As the water flows through limestone, it will absorb the nutrients from the lime in the stone, helping to keep your garden healthy.
One of the most important parts of the rock garden is the garden part. Like choosing rocks, choosing plants native to your region will keep your costs down, but it will also give your garden a more natural appearance. Planting seeds instead seedlings will aid in cutting costs and will give you a better knowledge of how the plant matures. Ask a nursery grower for tips on wild flowers which are native to your area if you are not sure.