Depending on context, sure this could be true. Like if you're comparing the cost of releasing some small OSS module of Uber's to Uber's annual revenue, sure, it's going to be a negligible cost. But if you compare against the cost of developing the module in the first place, it can easily be an equal expense. For example I spent several months on a BLE library for Android (https://github.com/iDevicesInc/SweetBlue) and asked my company to open source it. It took several more months to get the library to a point that was suitable for a proper OSS release. So cleaning up code, making sure no sensitive info, swear words and such, documentation, lawyery stuff, icons, PR copy, blog post, basic website, wiki, and much more nitty gritty I'm glossing over.
I mean a company can just throw code over the wall into GitHub and call it OSS, but I associate a basic level of polish with a proper OSS release that does indeed take a good amount of effort. And that's just initial effort! If it becomes at all popular then you have further ongoing overhead.
I'd say a general rule is that open-sourcing something is at least 50% of overall cost of the project.