The project cost of retaining walls Liberty Lake may vary depending on the materials to be used, labor and accessibility to the site here the wall will be built. Using pre-cast interlocking concrete stone blocks which lock together without using mortar is the most modern type of retaining wall developed. Its cost run between $15 to $30 or more per square face foot or approximately $750 to $1500 for 30 inches high and 20 feet long wall. A wood retaining wall can cost you $20 to $25 per square foot which place a 50-square face foot wall between $1000 to $1300.