How do you confirm that your business is receiving the lowest total cost from vendors?

When your business purchases products, services, and techologies, the lowest total cost does not necessarily mean the lowest price.  As you evaluate proposals from vendors, lowest total cost includes factors such as level of quality, level of service, the risk, and the price of the products or services supplied.  How does your business evaluate lowest total cost and do you find that it is challenging for small and middle market businesses to find the time and resources to conduct thorough vendor evaluations?