I am a corporate strategy consultant with 35 startups in my background as an angel and professional investor,m Founder, CEO, Director, advisor, or consultant.

The difficulty in answering this is lack of context and framing. If you were a startup with little revenue and significant losses that had paid € 1000 and now you are more mature with revenues at break even and the firm is asking € 3000, then you have one set of conditions. If on the other hand that cost was € 10,000 going to € 30,000 it is an entirely different set of facts.

In the US, we often have large accounting firms work at reduced rates with startups they like with the understanding that the rates will rise as the company thrives. They do not require anything other than a verbal agreement, i.e. they cannot and will not force you to stay if you choose not.

Finally, that you do your own bookkeeping is not relevant. It is expected that you would do so by all accounting firms. However, they do have to review all of that bookkeeping to summarize the financials because they will be at legal and brand risk should something implode; your bookkeeper will not be.

Hope that helps.