There is no obligation for applicants to pay any additional fee.
However, subject-matter that has not been searched will not be examined by the examining division (G 2/92). Hence, it cannot be prosecuted in an independent claim.
If the lack of unity persists also in examination after the claims have been amended, the excision of the non-searched subject-matter from the application may be necessary (see C-III, 3.2).
Non-searched subject-matter can always be prosecuted in a divisional application.