Source -Customers already know that the upgrade flavor of Windows platforms is much cheaper than the full version. This is not only valid for the general public, buying upgrade licenses of Windows 7 at retail, but also for volume customers. Traditionally, Microsoft volume customers opt to get their software through Software Assurance agreements with the Redmond company. However, Volume licenses of Windows 7 are available to business customers even without SA. And in fact, the software giant recommends that business customers buy the latest iteration of the Windows client this way, simply because it’s cheaper.
Windows 7 Professional Volume Licensing Upgrade Licenses Details - Courtesy of Microsoft - Softpedia