The biggest cost of a community shouldn't be the platform, you can (and should) start with something relatively cheap.
The biggest cost is the manpower. You need someone working full-time on the community. This is the big expense of community development.
The greatest tragedy is companies do the opposite. They spend a fortune on the platform and then cheap out on the manpower. They might leave the community to develop itself, recruit an intern to run the project, or ask a staff member to take on the community alongside existing work.
This simply doesn't work. It kills the potential of the community. It de-prioritizes the importance of the community.
And it will cost you much more money in the long-term.