Have a few people track how long they spent asking around and looking for a document (or answer) outside of the community.
You can use a survey if you want broad guesstimations.
Now, look at how long people spend getting answers or finding documents in the community.
What’s the difference in time saved? 10 minutes? An hour? 10 hours?
Once you take the time and effort to answer these two very simple questions, you can start to quantify just how valuable the community is (no. questions/resources found * minutes saved * avg. salary per minute etc…).
If you’re working to prove the value of an internal community, this is a good place to start.