The reason why parts cost more than the assembly is due to the fact that an individual part entails many more costs than an assembly.
Take a fuel pump assembly. Its a small box on the shelf. Its one SKU and easy to ID. Costs are low and inventory and handling costs are low.
Now take a check valve for the pump. They have to be bagged, tagged, documented, priced, and stored separately. You just doubled the costs for a business. One part vs two.
