§1. It is for the provincial council or a meeting of the bishops of the province to define by decree for the entire province the offering to be given for the celebration and application of Mass, and a priest is not permitted to seek a larger sum. Nevertheless, he is permitted to accept for the application of a Mass a voluntary offering which is larger or even smaller than the one defined.
§2. Where there is no such decree, the custom in force in the diocese is to be observed.
§3. Members of all religious institutes must also observe the same decree or local custom mentioned in §§1 and 2.
Source: Code of Canon Law (Vatican.va)