Because retaining walls vary so much in size, length, and design, it can be difficult to estimate the cost. Often, the price is calculated based on square-footage; that is, the surface area of the wall’s face. As a rough estimate, you’re looking at $25-35 per square foot at a minimum to cover materials and labour. So, a wall that’s 4 feet high and 20 feet long would start at about $2,000.
Of course, that can easily skyrocket to $10,000 or more if it’s a large or complicated project.
If you’re hiring a professional engineer to design the wall (generally a good idea), adding 10% to the price is a reasonable estimate.