We estimate the supply-side value of widely-used OSS is $4.15 billion, but that the demand-side value is much larger at $8.8 trillion. We find that firms would need to spend 3.5 times more on software than they currently do if OSS did not exist. […] Further, 96% of the demand-side value is created by only 5% of OSS developers.

— The Value of Open Source Software, Harvard Business School Strategy Unit, via Simon Willison