Answer:
One of open source’s biggest draws is that it does not cost money. “Free of charge”, however, is a byproduct of open source’s overall value.
Because an open source license requires that anyone can use, modify, and share your project for nearly any purpose, projects themselves tend to be free of charge. If the project cost money to use, anyone could legally make a copy and use the free version instead.
As a result, most open source projects are free, but “free of charge” is not part of the open source definition. There are ways to charge for open source projects indirectly through dual licensing or limited features, while still complying with the official definition of open source.