The cost of a retaining wall depends on several factors, including its height, length, and materials. Generally, the taller and longer the wall, the more expensive it will be. The type of material used also affects the cost, with concrete being the most expensive and wood being the least expensive. On average, a concrete retaining wall can cost between $25 and $50 per square foot, while a wood retaining wall can cost between $10 and $25 per square foot.