Maybe… almost universally, open source software requires more initial configuration work and more long-term oversight to keep operational, so if you’re making a statement like this you have account for additional labor costs. Proprietary software is usually sold as an out-of-the-box solution (it usually isn’t, but it’s usually a lot closer than open source equivalents).

The entry cost for an open source solution might be lower (no licensing fees) but the long-term cost might actually be higher, especially when you start trying to make various pieces of software work together. One of the areas where Microsoft does really well is system administration tools. Active Directory is a full suite of tools that all work together through a unified interface. To replicate AD you would have to patch many different open source projects together, some of which would overlap in functionality and some of which wouldn’t quite meet in the middle. As your environment increases in complexity and your sysadmin needs expand, these interoperation problems grow exponentially, which means more labor time and more expertise requirements, less stability and more security holes between the patched-together solutions.

Don’t get me wrong, I love open source software, but so far there are no good open source sysadmin solutions that scale well for organizations with thousands of users.