Portable storage Rapid City costs may vary depending on the size of the container that you will rent and the rental duration. The usual sizes of portable storage containers used in homes are 20 feet, 40 feet, and 60 feet. 20 feet portable storage Rapid City is estimated to be around $150 to $300 per month on average. A 40 feet container costs around $250 to $400 per month, and a 60 feet container is estimated to be around $350 to $500 per month. There are portable storage containers that have heating and cooling systems to regulate the temperature inside. You can also have a customized Rapid City portable storage for an additional fee.