What Sort of Investment?
As Kurt Vonnegut famously said in The Moon is a Harsh Mistress, TANSTAAFL - There Ain't No Such Thing as a Free Lunch. Everything, including Free and Open Source Software, has associated costs.
Before any discussion of Return on Investment, we need to arrive at a definition of required investment.
Products and Services - there are multiple ways to acquire open source software.
Download source code (or binaries) from a project repository, e.g., on GitHub, GitLab, or BitBucket
Purchase a product built with open source
Subscribe to a SaaS implemented with open source
Support - type of support and channels for it are determined in large part by the acquisition path.
Tools - while many excellent tools are themselves opens source, an organized adoption program can require
Training - as with support, options of training and the necessity for it are predicated by acquisition path.
Staffing -