It costs money because the publisher is a company for profit. The riff-raff is kept out by peer review, which is for free.

Many argue that research results should be free, resulting in what is called "open access publishing". The problem is that journals such as Science and Nature still have such a high reputation that the fact that a researcher is able to publish his researcher in these high impact journals has a strong influence on his success in applying for jobs and for research grants.