relatively speaking, that isn’t expensive for many serious forums and many well known organizations pay much more. please see my post here.
just a word of advice, consistently complaining about Discourse hosting costs and asking for exceptions to their pricing tiers for plugin availability will not endear you to them nor will it get you anywhere. many people before you have asked for exceptions and that is contrary to their business model. you are essentially asking for free services and work beyond what they are already giving you. most hosting companies don’t even have a free plan at all. moreover, the software code is open source and free to use, and a lot of their effort and time goes into keeping that code up-to-date, robust, and secure from vulnerabilities.