Length of contract and version details are very important too.
WebCT Vista for example seems to be around an order of magnitude more expensive than their legacy codebase WebCT Campus. They are currently backporting a great deal of functionality from Vista with the aim of totally replacing the current Campus technology. After their customers panicked they issued a statement that the Campus price-point would still remain even after they have totally replaced the codebase. However the rather radical changes to the underlying codebase mean you will now need, for WebCT version 6 an Oracle or MS SQL server licence and associated hardware and technical support.
The length of contract is important too as they generally try to lock you in for long periods but the price can rise dramatically when you come to renew the contract (I've been told recently of 30%-40% increases, not including the newer database licences)
I've kind of lost track of what Blackboard is doing but I believe they still do the same thing as WebCT whereby they limit the amount of integration you can do unless you pay a fair bit extra. When I last looked in detail at this the price would double or triple if you wanted the system to integrate with your student records database (i.e. that's not counting the consulting and development costs, just a cost for even being permitted to do it).
So I think to be truly useful the table should include the version (in as much detail as possible), how long the contract was for, and when the deal was done.