There are a few different ways to build a retaining wall, and the cheapest option will vary depending on the materials you have available and the size and scope of the project. If you're working with a small budget, one option is to use interlocking concrete blocks which can be stacked without mortar or other bonding agents. Another cost-effective method is to use pressure-treated lumber, though this will require more maintenance over time as the wood is prone to rot and decay. For a longer-lasting solution, you could consider using poured concrete or stone veneer which can both be very durable. Ultimately, the best way to save money on your retaining wall project is to do some research and compare prices on different materials before making your final decision.