In Germany, freelancers (Freiberufler) enjoy many freedoms. I am not required to pay for social security, I can freely chose between public and private health insurance, and it is not mandatory to use the services of a bookkeeper. In fact, the only bookkeeping that I need to do is for the department of finance: I keep track of income and expenses, I charge for VAT for customers in Germany, and I pass on the collected VAT. With business customers in other countries things actually get simpler: I don't charge for VAT at all.
I assume that for Polish citizens it's similar, meaning that there should be no additional bookkeeping required beyond what you are already doing. However, if you are working on-site in Germany, then after some months you have to change residency, and then the rules of your new home country apply. For example, during a recent stay in Spain, I was told by a fellow freelancer that he indeed has to pay for bookkeeping, and also for social security. He is an Italian who changed his country of residence to Spain.
In a nutshell: Bookkeeping costs vary greatly from country to country, and in the west they are not necessarily more expensive than in Poland.
I want to add: Health insurance can cost hundreds of Euro in Germany, and it is mandatory at least for German citizens living in Germany. Some years ago, for freelancers health insurance was not mandatory, i.e. you could decide to go uninsured, but that has changed.
