There is no definitive answer to this question since there are many factors that can affect the cost of a retaining wall, such as the length of the wall, the height of the wall, the materials used, and labor costs. However, a simple concrete block wall will generally be one of the cheapest retaining walls to build.

The cheapest retaining wall to build is usually a simple earth/gravel retaining wall. A trench is dug and a layer of gravel or stones is put in at the bottom. The soil is then back-filled against this layer, creating a retaining wall. One of the benefits of an earth/gravel retaining wall is that it can be easily increased in height if needed, simply by adding more layers of gravel or stones. However, care must be taken to ensure that the weight of the soil does not exceed the weight that the retaining wall can support.