This is a highly contented topic. Vendors of proprietary OSes often claim that even though their products are not free, the actual cost of ownership is lower than an open source product like Linux. Linux proponents, on the other hand, argue that the cost of ownership of Linux is lower than that of proprietary OSes.
The truth is that there is no easy answer.
The cost depends mostly on staffing, salaries and the use of consultants for support purposes. For example, if an IT shop relies solely on consultants or vendors to install and maintain systems, then the saving of licenses will be buried in the cost of consulting and contracted support.
On the other hand, if an IT shop has an in-house team of experienced Linux administrators and/or developers, then the saving from the cost (or there lack) of licensing is significant.
It is not the purpose of this module to analyze the TCO (total cost of ownership) of Linux and proprietary OSes. It is important to remember that the TCO of a system depends much on staffing and management decisions.