Hello rheejeff
Flarum is open source. Using this software is free of charge under the MIT license. You don't need to pay for anything while using this software, except for any fees associated with your hosting environment.
Please note Flarum is still in beta and even though many people already have it installed on production environments, we still give off a slight warning that an upgrade path to stable might require you to get assistance or do some manual dirty work (in your database for instance).
If you have any further questions, feel free to ask!
