Although this blog entry is about capital costs, this is just the thinest edge of the cost wedge: the big dollar differences are in operating costs and user productivity - things I'll start to get to tomorrow.
The big differences in cost come from software licensing, the accommodations you choose to make with respect to failure, and the decisions you make with respect to the minimal hardware needed to do the job.
To start with the easiest stuff first: the list price cost to provision a thousand Sun Ray desktop users with 24.1" TFT screens mounted on the standard 2FS Sun Ray with Unix keyboards and mice, is $1,219,000.
Doing the same thing with Dell's Optiflex 755 (1GB, Core Duo @2.53Ghz, 22" LCD, and the "Ultimate Vista downgrade" to XP Pro pre-installed) will cost you $1,311,000.
On the server side, if your base load is Office, E-communications, and file management, you'll need a rack of eight Solaris servers: six of them dual core T5240 processors running in 64GB, two X4500, 24TB, data servers, and a shared one hour UPS for about $320,000 at list inclusive of the Star Office and related software you need.
To ensure both operational continuity and appropriate levels of resource over-kill you'd get two sets, put them in different places, and assign them to separate sysadmin teams for a total capital cost, at list, in the neighborhood of $650,000.
On the Windows side the server picture is much more complicated largely because the key redundancy strategy is based on layering virtualization over clustering - for example you'll need around eight exchange servers but they could run on only four machines and would typically all access the same SAN. You can choose, furthermore, between additional virtualization on larger machines or greater redundancy on smaller machines - and, of course, all the software has to be licensed separately.
One reasonable approach would be to use 18 Dell R900 2.83Ghz E7220 servers with 16GB each and a single shared 24TB Dell CX3 SAN with VMware and Microsoft's 25 CAL, x64, enterprise server 2008 pre-installed on each - at about $460,000 inclusive.
In addition you need a thousand Office Professional 2007 licenses at a retail price of $329,950 - for a system total capital cost, at list, of about $790,000.
Thus the system total for the fully redundant 24" Sun Ray solution comes to about $1.9 million or $1,870 per user while the comparable numbers for the 22" client-server system are $2.1 million or $2,100 per user.
That difference is small - $200K would matter to me personally, but should not be a significant factor in making a decision between dissimilar systems for a business employing a thousand people.
So what will make the difference?
The big capital cost difference will come from the primary non Office application, or applications, you want to run on those desktops. As configured the Sun system is overkill with full redundancy and more of everything from screen real estate to storage and processor power -so adding something like Oracle financials would just add the licensing cost to the set-up. In contrast the Windows system is tight because most of the memory, processors, and storage you're buying is on the desktops and so useless when you want to add significant shared applications - for Oracle financials, for example, you'd have to add more servers and storage along with the new license fees and then face significant backup and redundancy limitations.
Generically, however, the long run biggest cost difference is going to come from staffing - the Unix system will need five or six IT people, the Windows one 20+ IT people and significant hidden commitments of user self-support (aka futz time).
Politically the biggest difference is in power use. The base 2FS Sun Ray uses 4 watts: two thirds what Dell's Optiflex uses in sleep mode and about 2% of what the 1GB model requires in full operation - and bear in mind that every watt converted to heat in the machine adds another watt equivalent to the building cooling load.
Note that, in reality this cost difference is trivial relative to staffing cost differences, but people are really bad at pricing political correctness and 4 watts + screen vs 180 Watts + screen makes for a really compelling overhead - and you get to explain what "in perpetuum" means.
From a capital cost perspective, however, the killer issue is longevity. Roll out those Dells and you're into the nearest approximation yet invented to perpetual motion: the evergreen cycle under which users start to complain about the limitations of crappy out of date gear as the installer leaves their cubicles. Roll out Sun Rays instead and you're set until those 24.1" screens become obsolete - probably around ten years anyway.
Capital cost is easy - but fundamentally much less important than people think it is. The big issue, dwarfing even staffing costs, is the differential effect on user productivity exercised by the two architectures.
I'll start on that tomorrow but, for now, think about one minor aspect of this as it affects IT operations: server OS upgrades in the Solaris world aren't usually visible to users at all; and application upgrades are visible only to users of the affected application, have no other effects on their desktops, affect all, or some subset of, users at exactly the same time, and can (usually) be rolled back with little effort.