You get what you pay for
That is how the common idiom goes, right? The meaning being that the price of something equals its quality. But does it?
Open Source projects are often provided at zero price and yet some of the most reliable and highest quality software products we know of are Open Source. You can build highly advanced and yet reliable things and applications based solely on code you pay zero dollars for.
Time is never free and you also need computer equipment and maybe other infrastructure to run the Open Source so no one gets away completely without paying anything, but you can avoid paying the authors of the software and yet get to use high quality code without breaking any laws.
Even if you can get away without paying anything to any creator of Open Source, it might be worth considering hiring someone from the project or getting a support contract for it, to drastically reduce the amount of time you waste getting lost by not understanding how to use the code most efficiently etc.
Maybe you do not extract the optimal value from the project by not paying anyone knowledgeable and therefore the product you build with it does not come to its fullest potential.