Filner is scheduled to resign Friday following sexual harassment allegations from more than a dozen women. He was San Diego's first Democratic mayor in decades when he took office in December.
U-T San Diego reports the city charter requires a special election if a mayor resigns with more than one year left on his term.
The city clerk estimates the election would cost about $6 million and more if a runoff is required.
The U-T said it's likely that a crowded field of candidates would prevent anyone from receiving a majority of the vote. That would necessitate a runoff between the top two candidates.
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