Is there a legally mandated filing fee and/or deposit?

See Parliamentary and Presidential Elections Act 1993, 114 (2). The law only says:  On hearing a petitioner subsection (1), theHigh Court—

(a)                        in its absolute discretion, may or may not condemn any party

(b)                       to pay cost in accordance with its own assessment of the merits of thecomplaint. To the Supreme Court of Appeal, the law is silent.

http://aceproject.org/ero-en/regions/africa/MW/malawi-parliamentary-and-presidential-elections/at_download/file