This depends on where you are building the wall and what kind of wall you are building. There are several types of retaining walls and the costs will vary.
Anchored retaining walls are secured at one or more points by cables. The cables are usually anchored in the soil or rock sitting behind the wall.
Sheet pile or piling retaining walls are thinner than other kinds of retaining walls. They are made of long vertically set beams or wooden planks and one-third of the length is usually buried into the ground.
Cantilevered retaining walls are usually made of concrete or masonry that has been poured and set in place. They are shaped like an upside-down T. The footing of the wall is set underground and provides additional support against the pressure of the soil being retained.
Gravity retaining walls rely on their own mass to withstand the pressure of the soil behind them. They are built with stone or concrete and are best for domestic situations. The type you choose depends on where you live. It's a good idea to consult with a contractor or other professional to see what they might think is the best. Some materials might be better for your unique situation than others. The price can range from about $137.50 to $250 per square foot. Some contractors might instead ask you to pay them an hourly rate rather than paying for the wall. In this case, they will likely charge about $55 per hour. In some cases, you might also need to hire a construction engineer to check the area and boundaries. This will be an additional cost, ranging from about $120-$180 an hour. Sometimes after the job is done, you will also need to get additional soil. If you are worried about the way the wall looks, you might also want to spend some money buying plants or doing landscaping to make the wall look more appealing. This isn't necessary of course and is a personal preference. Once you have decided what kind of retainer wall you want and the materials you want to use, ask the contractor for an estimate of the final cost. This will give you an idea of how much you need to finance or budget.