I have taken apart my HP and I know that they have a parts supply site that I can order replacement parts from.
If you have a center trackpoint thing in the keyboard, that sounds like you either have an IBM, a Dell, or one of the Toshibas that has that.
I have no idea on the cost, but I know that for me to replace the keyboard on my laptop would have cost $40 and then my own labor to replace it.
If you take it somewhere locally, then they ideally they have the part in house, if they don't then they can dick you on the shipping and the time involved.
Worst case scenerio is that you have to send it back to the company to fix - that can take a long time.
The cost itself is likely going to be around $100 on the low end.
If you don't actually travel much with your laptop, or if you don't actually use your laptop on your lap and usually have it on a desk/table in front of you - then you should be able to just attach a mouse to it via the serial port or USB if you have it.
That would be a much cheaper solution.