Chris covered everything very thoroughly, but just so there's an "official" response:
Yes, when the game is finished it will be released to everyone for free - no paygated content or purchasable DLCs or anything like that :)
What I will do, however, is also release - at the same time - a completed unencrypted and uncompiled version of the game for a small fee. This version will be released under the Creative Commons license, which will allow you to do anything you want with the code, characters and assets.
Want to mod the game? Sure thing! Want to use it as the base for a brand-new game of your own? Absolutely! Want to take the characters and use them in other forms of media? You go, girl. Whatever you like. I'd honestly be fascinated to see what people do with it :)