The amount of the collection costs depends on the amount of the original bill - and the collection company's scope of the work. In principle, however, the debt collection companies must not charge more than what a lawyer could do for the same job. The amount of money a lawyer can charge for certain activities is regulated by law.
On the website of the Consumer Protection Centre, you can find the so-called "Inkasso-Check", where you can check whether the amount of the collection costs required is justified. If the collection costs are too high according to the "Inkasso-Check", seek help from a Consumer Protection Centre. You can find a consumer Protection Centre nearby on verbraucherzentrale.de.
Please note: If you have forgotten to pay an open invoice, but have not yet passed the deadline, you do not have to pay the warning letter costs and collection fees. If that is the case, only pay the open invoice and inform the debt collection company. You can find a sample letter on the website of the Consumer Protection Centre: Click on "Inkasso-Check starten" and enter all relevant data there. In the end, you will receive a fitting sample letter you can send to the debt collection company.