Having recently completely a filter comparison document for some independent schools I found there can be many influencing factors that all relate to cost in the long run. For instance as ash mentioned above licensing models can vary greatly, appliance v software solutions x the amount of sites needed to be filtered, and of course ease of administration can really effect TCO and filter effectiveness, which influence the total cost to the school / district.
Perhaps the most interesting area was like storage and virtualization the fact that a lot of security solutions can now be consolidated into all in one solutions or UTM's. By converging several related solutions such as Filtering, Spam, Shaping, AV, and firewall functions greater savings can be made over a period of time and lower TCO's.
There are lots of solutions out there, some more education focused than others and some will suit some enviroments better than others...but its worth looking at all the options. Most popular isnt always the best for your particular needs, it can just mean they have been around the longest....eg MS
Just my personal findings anyway.