Certain things have minimum fees, or hire periods, regardless of the amount of work done for the specific job, even when that task is charged by the amount of time spent (e.g. dubbing, editing, filming, etc.). This covers things like set-up time, equipment wear and tear, the impossibility of scheduling multiple jobs into very small time periods, minimum hire periods for labour, etc.
In some cases the charge will have to be more than the minimum cost, such as jobs that entail travelling expenses. The minimum fee is the minimum fee that may be charged, not the lowest amount that will be charged. What a job actually costs will always depend on what actually needs doing. Ensure that you organise your job well, in advance, so there won't be any surprises.