It is commonly believed or assumed that Open Source is free of charge. The reality is that in general almost all Open Source Software can be obtained via a free download from the internet.
Open Source Software released as a GPL licensed software requires that it is distributed with no restriction, and that the code source must also be made distributable similarly. Furthermore, the GPL (which is a legally enforceable license) requires that the same rights be provided to all derivatives and subsequent copies of the original software.
This does lead to the fact that Open Source Software is available in infinite supply. The laws of economics will dictate that a product available in infinite supply will be free of charge, apart from costs associated with the process of distribution (ie: internet infrastructure: servers, hardware costs, bandwidth costs etc).
As a result Open Source Software is therefore available at very low cost.
Note that this is a consequence of the licensing model under which the software is distributed and not a direct consequence of the development process of the software. The point is that the fact that Open Source software is freely available has no bearing on the quality of the software and fitness for a particular purpose. Open Source Software can be as good as any proprietary software and very often better.