While the cost of storage devices such as hard disk drives continues to fall, the overall proportion of computer network costs dedicated to storage continues to rise. Within a few years, storage will account for 50 percent of total network hardware and software costs. Since computer networks typically have long lives, the design process often involves projecting costs many years into the future. This paper examines a rule based on a technology trend that can be used to estimate the cost per megabyte of hard disk drives. The rule is similar to the well-known Moore’s law that has reliably summarized integrated circuit advances over the past several decades. A statistical analysis of historic data suggests the rule for hard disk drives captures much of the readily available information.