Clusters can be built with high-volume components, which are relatively inexpensive. Relatively few high-end SMP machines are sold—relatively few customers will buy a 20-way SMP. So, 20-way SMP hardware systems are expensive because the engineering cost is amortized over relatively few units. Inexpensive clusters can be built with commodity 4-way SMP hardware systems. This means they have superior price/performance. In addition, because they are built from commodity components, commodity clusters can grow in smaller increments. You can add disks or nodes or network cards as needed rather than buy a huge new box each time you add to your system.