The 10.0 to 10.1 upgrade was sort of free but sort of not - you could buy an upgrade pack for something like $20 or $30, or I think there was some way you could get Apple to give you a burned disc of the update for free, as opposed to the proper disc and pack that you got if you paid. The update was never available for download, I suppose because back then the large file size wouldn't have been a realistic download for most people. The upgrade disc can be used to install 10.1 without an existing 10.0 installation by removing the package CheckForOSX from the disc and re-burning it, I think (Google it for the full details). Though I think later revisions of the disc had this route blocked, as Apple were furious about the discovery of this workaround.

If you want to get genuine copies of 10.0 and 10.1, you can actually get them from eBay without spending any money - I have full retail (non-upgrade) boxes of both, just because I wanted to add them to my Apple collection, which I got by buying them and then selling the OS9 discs that were included with OSX in those days (for Classic) for more than I paid for the entire set, thereby actually making a profit and still having the OSX boxes! I've never actually installed them though (I have the Public Beta set too) - I'd like to have the space for a second G3 tower for trying out all this kind of stuff! (the G3 I have is kept as a stable machine for 9.2.2 and Tiger - I don't want to risk screwing up the setup)