There aren't any current TCO studies for MacOS X or 2000 ... neither has been in the marketplace long enough for any accurate data.
Generally speaking UNIX servers need less than half the amount of system admin time as NT server. The big win with UNIX though is that full remote administration is included out of the box, so you need less on site people. WTS is an additional cost for NT, but is included in a some form in Win2k.
The down time for UNIX boxes is usually MUCH lower as well ( ie hours per year versus days-weeks per year ).
On the desktop, you need about 1/4 the amount of desktop support people
for Macs versus NT. Generally speaking Mac hardware in a big deployment works out at about 1/3 cheaper over a 3 year life cycle than Wintel Hardware.
It works out better over a 5 year life cycle, which is usually viable on Macs if you buy the right hardware, but usually isn't viable on Wintel.
Your mileage will vary depending on what you are actually doing , and what your users do.
I would avoid Win2k for the server back end for Macintosh at the moment, there are some (major) bugs in their Networking and Thread Managment that make Win2k problematic for both file serving and authentication managment . They remain unfixed since the initial release of Win2k. There are other issues like ADS not playing good citizen with OpenLDAP etc, but that may or may not be an issue for you.
Having said that, MacOS X server is not much better right now, but at least
Apple has a strong incentive to fix their server issues. I'd wait until the 10.1 version of Server ships, and then evaluate it, or maybe do a pilot deployment with it late this year.