The 1.4 release is free of charge and always will be.
If you are using DOCman 1.4.0 or older:
As promised, DOCman 1.4.x will always be free of charge, and is still receiving security updates. However, it is no longer officially supported, and will not receive bug fix updates. There are two options:
* Update to DOCman 1.4.1 for free
* Upgrade to DOCman 1.5.8 for the price of lunch and a coffee. Upgrading is painless and preserves your data. You will get updates to all 1.5.x releases, as well technical support. And of course you are supporting a quality GPL-licensed Joomla extension!
Please see the README for upgrade instructions.