An ROI model is necessary to calculate the monetary value of your community for your business.
There are several ways in which a community provides cost reduction for the business and thus demonstrates ROI.
Measuring Decrease in Customer Support Tickets:
When you enable a self-serving community, then you have empowered the community members to get their queries answered within the community.
This results in monetary savings as it relieves pressure from your customer success team.
You can calculate the exact monetary value by finding out the cost per Support ticket and multiply it to the decrease in the number of support tickets to find the ROI.
ROI from Share of Voice:
When you build an engaging community, your community members spearhead your brand presence, providing you cost reduction in terms of PR.
Customer Effort Surveys:
In a QnA session with Adrian Speyer (Head of Community, Vanilla Forums) in the CMX Summit 2020, he told the benefits of Customer Effort Surveys.
You can run CES surveys, which are essentially pop-up surveys where you ask your users were they able to achieve what they wanted to achieve from their visit to your community support resources.