San Francisco’s over-budget and oversize $2.4 billion Transbay Transit Center will open in December — but it’s going to cost an estimated $20 million a year to run the place, and no one knows where all the money will come from.

The three-block-long behemoth was envisioned as the Grand Central Station of the West, a dynamic hub for buses and high-speed rail that would draw more than 100,000 visitors a day.

Come opening day, however, there will be no high-speed rail. Instead, for many years, the five-level showcase just south of Mission Street between Second and Beale streets will be little more than the world’s most expensive bus station — serving mainly the 14,000 transbay bus commuters who roll in and out daily on AC Transit.

That reality is starting to sink in and has city officials scrambling — because without the big crowds that trains were supposed to bring in, there are serious questions about where all the money needed to keep the place secure, clean and well lit will come from.

“The elephant in the living room is solving the operating subsidy problem, which could be as large as $20 million a year — and without a source of revenue,” said Supervisor Aaron Peskin, who chairs San Francisco’s Transportation Authority.

With the transit center expected to stay open around the clock, officials say it will take at least 65 private security guards — plus police and sheriff’s deputies — as well as a staff of janitors, maintenance workers and others to keep the place from becoming a giant homeless camp.

Taxpayers and bridge commuters will probably be on the hook to pick up millions of dollars in costs, although the exact amount still isn’t known.

“We expect to have an operating deficit,” said Mark Zabaneh, executive director of the public Transbay Joint Powers Authority, which is building the center.

Business software giant Salesforce has offered to kick in about $3 million a year in return for putting its name on the center and the 5.4-acre public garden that will sit atop it — but City Hall sources tell us that deal has yet to be sealed.

The transbay authority is also hoping to collect more than $1.5 million a year in fees charged to the developers of high-rises that have sprung up around the center.

The authority also hopes to bring in cash from rooftop garden events, as well as from advertising in and around the center.

Bridge commuters, who have already kicked in $350 million to help build the center, will probably be contributing more toll dollars to run it. The transit authority will probably ask for a piece of a toll-increase initiative that could go on Bay Area ballots next year.

“We will be seeking additional toll money,” said Zabaneh, declining to say just how much drivers will be asked to chip in.

Meanwhile, the authority has been working for months to find a master lessee to run the transit hub, and to line up tenants for the 100,000-square-foot mall that will occupy a good portion of the building.

“We are looking for a team that will give us the best offer, with a good profit-sharing margin and zero cost to us or the taxpayers,” said Mohammed Nuru, the authority’s president and director of San Francisco’s Public Works agency.

But there’s a problem.

Without the foot traffic that high-speed rail could draw, the mall is looking a lot less attractive to potential renters. That means the authority may have to offer sweetheart deals to lure stores — which, of course, means less money.

Zabaneh said the authority expects to name a master lessee to oversee the transit center operations when its board meets Thursday — at which time, “we will release all the data we have regarding the expected operational costs.”

The transbay authority is also preparing to ask AC Transit and other bus operators — including Golden Gate Transit, Greyhound and Amtrak — to pay millions in rent. That’s on top of the $5 million a year in toll money that’s already used to cover costs at the temporary terminal downtown.

“We will be asking the operators to pitch in,” Zabaneh said.

Robert Lyles, spokesman for AC Transit — whose 450 bus trips across the bay each day are expected to account for two-thirds of the new center’s bus traffic — said the agency is ready to pay its “proportional share” of operating costs. But he said no one has given AC any numbers.

San Francisco Chronicle columnists Phillip Matier and Andrew Ross appear Sundays, Mondays and Wednesdays. Matier can be seen on the KPIX TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call (415) 777-8815, or email matierandross@sfchronicle.com. Twitter: @matierandross