It depends on your level of expertise and the type of tool. For instance, Sphinx to write documentation is free, but you need to have at least a desire to write in markup to create it, and I'm assuming that single-sourcing multiple user manuals with conditional text and variables is not exactly trivial. Plus, sharing the writing of such a user manual between more than one person requires the use of a third-party solution, a version control system, which also requires some training & expertise to use properly.
Where as, on the other hand, if you can convince your boss to dish out $200 a month per user for something like Author-It, you get an extremely powerful system that does everything an authoring tool is supposed to. That's what I'm trying to do right now with my boss, convince him to switch (not from Sphinx, but from another cheaper software solution called Flare), and it's not an easy task.