I call BS on these figures to a large extent. I currently work as an IT consultant to the small/medium business space including many your size. I'll preface this by saying I am an RHCE and really love linux personally and part of me wants to try to sell it (in the from of the free CentOS) as a solution more often. Even with a openldap/samba/zimbra solution it just doesn't measure up with MS SBS which is a great product though...
My first problem with your story is how much of your time did you spend on this? Would your average small buisines owner have that time or required interest/skill? If you had to pay somebody to set up the Linux solution your costs would have blown out considerably. My second is that you don't have to use all pay software on Windows - Open Office and GIMP run on Windows just fine...
SBS 2008 standard which includes Server 2008 (with the requisite AD/Group Policy goodnesss), IIS7, Sharepoint and Exchange 2007 is $779 including 5 CALs/licenses. Dell/HP servers it might even work out cheaper because it is bundled.
It then costs ~$70/user in additional CALs beyond the 5
I usually drop in an high-speced HP ML350 server with 4 cores of Xeon, 8GB ECC RAM, 4-5 10k SAS disks in a RAID5, LTO3/4 tape drive, redundant power supplies and integrated lights out management (lets me reboot/work on things remotely) since they'll be running one server - in most cases for organizations of this size one really great server is better than a few mediocre ones anyway.
New PCs are quite affordable these days and come bundled with windows (I still get them with XP for the most part) and you can get Office 2007 small buisiness for $239/copy.
I'd say all told this costs them $15,000 for the server including all license costs, hardware and setup and then $1500/desktop for the all hardware/software/setup costs including XP and Office 2007 (many still exercise downgrade rights to 2003). Exchange/Outlook/ActiveSync is the current gold-standard for messaging/collaboration and hooking their iPhones up takes moments and has been an easy sell.
Server 2008 and Exchange 2007 are surprisingly great products and now that I have mastered powershell I can manage/script almost everything from the command line - who'd have thought on MS? I stop in for a couple afternoons a month to do desktop/user support and perform ongoing server maintenance. The maintenance is mostly chasing up the backups and making sure they don't run out of space - I have yet to have a major issue with SBS 2008 and had very few with SBS 2003...
So for ~$60,000 total including all licensing and setup costs they have the full MS solution in an organization of your size. Wack on another $2000-$2500/month for my visits and there is the bulk of their IT costs. I have yet to have any complaints with it and since it is a standard solution built to MS best practices and well documented I am sure they can find a guy to take over from me without any problem if I leave too...