Open source doesn't even mean it is free. It simply means that the source code is available, openly.
You need to see what kind of license it has, and read the license. It may have it's own unique license, or may use one of several common licensing models.
At minimum, by definition, the license would have to permit you to download and examine the source code. If it permits nothing else, then it is still "open source".
The minimum is uncommon, but has it's uses. For example, imagine a commercial cryptography application. The manufacturer might want to make the source code freely available to experts world-wide to verify that it does what it says and is not flawed. But they still may require payment for any use of the application. (Except, perhaps, for testing it for flaws, backdoors, etc.)
I think slashdotters coined the phrases that most accurately make the distinction:
"Free as in beer"
"Free as in freedom"
A package can be free as in beer (e.g. cost-free) or free as in freedom (you are free to look at it, perhaps to alter it), both, or neither.
A free-of-cost package may or may not be open-source.
An open-source package may or may not be free of cost.