Today Linux OS is highly demanded in the market. It is known that Linux is almost free and more less affected by virus or almost zero. Modern Linux has capability to do all daily works and server admin jobs that all other OS(s) do. Needless to say that, Linux has several applications built-in that are not necessary to install after installing Linux. The only lack / problem is that, there r not so competent persons that handle Linux easily. That is the main problem that people go for hire Linux experts and expense lots more than other OS. Try to establish an in-house training facility on Linux system providing technical books and howtos and also try to make habit to work on Open Offce instead of MS Office (equvalent to MS Office and it comes free with Linux). Try to get familiar with user level daily activities and their daily works. I am astonished to know that, only due to this reason can anyone cancel migration while, it is universally known that, Linux is Unix like OS or almost all commands and tools can work on Linux which work earlier on Unix OS. Ur work is more easier than my situation…. a years before, my org. decided to migrate all windows (both desktop & servers) to Linux (SuSE) and we done it very happily and smoothly. Yes, we faced a lots of trouble at very initial stage, but slowly but steadily first we created a work force by selecting staffs, who r more familiar with command line tools within the org. (hope, you know that there are so many command line utilities in Windows OS, but we never use or use them rarely cause there r GUI tools to use the same) and did the job successfully. Believe me, we never take any outside supports. We did R&D process in test computer and after getting success we implemented the same (still it is on going). Not only that, we read several books on linux, articles, news, attended seminars and so on. After several months, hope you and your team also will be expert on linux systems. Yes, of course, you will take time from your client or company to migrate the same.
For your better reference plz navigate here for Cost effective Server and 3rd party cost related.
Though I am totally Linux centric (specially on SuSE) you can also try for RedHat.