What you need to do is factor everything into the equation. That includes not only the OS, but your support costs, training, hardware, your development environment, etc. I also wouldn't assume the same level of servers, either. Generally speaking, you may have more Linux servers on your site than Unix servers, as the hardware for Linux is usually Intel PCs, which are not as powerful as RISC Servers.
Of course, the top Unix vendors today -- HP, Sun and IBM -- will all let you run certain versions of Linux on their hardware. The truth of the matter is, you'd be hard-pressed today to find a lot of folks actually doing this, so I'm not convinced yet that the hardware vendors are not speaking a bit between both sides of their mouths. If Hewlett Packard had more people using Linux on their machines than HP-UX, guess what? They'd be losing money on their bread & butter operating systems, which still account for nice revenues. The same holds true for IBM and Sun. The day I see an announcement that a fortune 500 company has starting using Linux as a replacement for AIX on an IBM pSeries using logical partitioning, then I'll start believing in the words more.
Anyway, I'm getting off the subject and since I tend to listen to my customers, I will assume that the same levels of servers would be used. There's little problem with storage here; in most cases the storage would be identical. We know the operating system will be cheaper on Linux, but what about the hardware? Linux wins again here: You can get a decent server for a couple thousand or less, whereas equivalent offerings from IBM, HP or Sun can cost in the tens of thousands, if not more. As far as OS and driver support, here is another case where Linux is cheaper. Generally speaking, the enterprise type customer will need a support/maintenance contract from the Unix hardware vendor. With IBM, there is a separate contract for Unix and Hardware. This can be very costly, but necessary, as you will need the vendor help to install patches and help you with drivers and other issues. With Linux, again, you need no contract. You support yourself and get the patches and drivers either from another Linux variant for a small fee, or through open source contacts in that community. I would say the initial costs may be greater with Linux, as a person with Unix will receive more hand-holding from the vendor. Comparing apples and apples again, though, same level of experience Linux or Unix administrator, then again Linux comes out ahead.
As far as software development is concerned, I would say there's probably a wash, though licensing is a different story. You will pay for everything on Unix servers, while you can get a lot of free Linux software that does similar tasks. Even where you must pay for licenses, when you compare the cost of Oracle or DB2 licenses on Linux vs. AIX or Solaris, it will be cheaper on Linux platforms.
Bottom line, everything being equal (and I must say, it rarely is), the total cost of ownership for enterprise solutions will be cheaper on Linux than Unix. It may be messier, more difficult to set up initially, and the apps might not be as stable while you are first tuning them, but in the long term you can do a solution that works more cost-effectively.
There are some good white papers available on the net.
FOR MORE INFORMATION:
Best Web Links on the cost of Linux
This was first published in October 2003