Before the mandate: discuss the fee openly, the RATG and AHK or a fee agreement as the basis.
The fee of a chosen defence is governed by the Lawyers Tariff Act (RATG) and the General Fee Criteria (AHK) or by an express fee agreement. Which costs actually arise depends on the scope of the proceedings, the number of hearings and the effort of the defence.
From a legal perspective it is sensible to discuss the fee openly before giving a mandate. That way you know what to expect and can make a deliberate decision.