The main factors as I see it are:
At a glance Linux seems fit to replace Windows in majority of desktops, because they usually run (computer role):
- Only general programs (Office, E-mail, Web)
- Graphical software (Photoshop, inDesign, 3DsMax, Autocad)
- Programming software (Eclipse, NetBeans, MS Visual Studio)
majority of those can be easily replaced by Linux alternative (The Linux Alternative Project, Linux App Finder). And then we also have wine or vmware with whose we can bypass the rest. For example following roles:
- IE only web applications (these are fading)
- Finance & controlling programs (SAP, MS Money, MS Project, Accounting software)
- Specialized, but generally available programs.
- Tailor made program only for one company.
There are also emotional factors which make people stick with Windows:
- Employees training
- OS specific applications (people don't want to give up one application in favor of its alternative).
- Strength of owner of the business/CEO overcome these factors in imperative way.
So what are the financial factors?
- How much the company pays for OS licenses?
- How much the company pays for core programs (Office, Graphical programs, Programming software) and Security software (Anti-virus, firewall)?
How these expenditures repeat in time
Of course there are some things which put the prize of Windows solution down
- New computers (mainly) don't sell without pre-installed Windows
- Unauthorized usage of software (software piracy) - puts cost of SW to zero. Helps to increase Windows market share. Thus actively works against free software.
And also costs of having Linux
- (Optional) One time migration cost
- (Optional) Paid commercial support
- (Optional) Employees training
- (Optional) Need of system administrator
How much they can save? It is enough to motivate them to go through the trouble of migration?
- Mexico city government uses Linux
- Municipal library, Prague
- (Irrelevant) Android, IOS, servers, TVs DVDs and media centers
- The cost savings might not be big enough to motivate the migration.
- Most of the desktop computers should be fit for the migration.
- Emotional factors play significant role as a barrier for the migration.
Added need of system administrator among costs as suggested by @Austin .