There are a few factors that affect the cost of a retaining wall, such as the height, length, and material of the wall. Generally speaking, a concrete retaining wall is the cheapest option, while a stone retaining wall is the most expensive. That said, there are many DIY options available that can be significantly cheaper than hiring a contractor. For example, building a simple timber retaining wall is a relatively easy project that can be completed for around $50-$100.

There is no definitive answer to this question since it can vary depending on the materials and construction methods used. However, some of the cheapest retaining wall materials include poured concrete, earthbags, landscape timber, and railroad ties. One of the cheapest ways to build a retaining wall is by using stackable concrete blocks. You can also purchase a precast concrete retaining wall system which is relatively affordable. Another option is to build a retaining wall out of natural stones or boulders which can be expensive but also quite beautiful.