Discourse is actually open source and free. You can download all the files and install them yourself if you know your way around linux.
The price that you quoted is only if you want to run it on their servers. We're not running it there, but on digitalocean which is $20/month .
You'll also need an account that handles the emails from mailgun or a couple of other providers they recommend. Depending on which you choose first 5000-15000 email will be free each month.
It's a little different that phpbb and vbulleting, you'll need a little linux knowledge, but I'm starting to like it more and more, a lot of well though features and it's more oriented on creating a community unlike the regular old forum platforms.