Answered here
https://www.discourse.org/pricing.html
What if none of these plans fit my budget?
Good news! Discourse is, and will always be, 100% free and open source software. You can install it yourself on your own server, or for a flat one-time fee of $99, the community can install Discourse in the cloud for you. Please note that a $10/month hosting fee is still payable to the cloud provider (Digital Ocean), and that this option is completely self-support after the initial install. Click here to purchase a self-supported community install.