It's not just cost. It's the freedom to not be forever tied to a properitary vendor's licenses and changing business model.
The changing licences is a problem for FOSS too - if a FOSS project you rely on goes from LGPL to GPL that can make a difference to whether some companies can use it.
If you want to keep your system secure, you'll need to have a plan to update all of the stack beneath you for security upgrades as well (whether that's Windows or Linux or whatever), and so you're not immune with any vendor from changes of direction, or licences by the technology teams beneath you.
The fact is that unless you build the entire technology stack beneath you, your business is partially dependent on someone else, and that can come back and hurt you whether you're dealing with a small company, a big company or an open-source community.