Linux

It can vary, but there are multiple licenses at the enterprise level with varying agreements and costs. Not just the OS for your server, but software, services, end user devices, and other random things that most folks never think about because they don't have to.

In some cases FOSS can take a big chunk out of those costs or even eliminate them entirely if you have good staff that knows their stuff and your business doesn't need or can make it's own niche software/systems. If you build it in-house, you have to support and maintain it but it's still often cheaper than many paid solutions.