The development costs will be a one off...
Having maintained windows, linux, solaris and novell based networks my experience is basically...
You require competent staff to manage any system properly, microsoft marketing says otherwise so windows networks often end up being operated very badly by incompetent staff (and have major security and stability problems as a result)... Generally only more competent people even know linux exists, so the cheaper less competent staff will never even think to try linux - if they did the results would still be bad but probably not as bad as a poorly deployed windows setup.
If you don't mind a poor setup, windows will cost more than linux but you will probably not be able to find as many extremely cheap low skilled staff pretending to have linux skills as windows...
If you want a good secure linux setup you need decent staff...
If you want a good secure windows setup you not only need decent staff but also a lot of third party software...
It's also my experience that you need more staff to maintain a windows setup unless you cut corners...
The problem is corner cutting, people think they can cut corners with windows but the end result is a huge insecure mess.. 99% of the companies i've ever been to simply don't have the budget to maintain a windows network properly..