OAKLAND -- A federal study now under way finds the cost of building the new Bay Bridge -- specifically the signature 525-foot suspension tower -- could explode again, this time by almost half a billion dollars.

Almost ten years ago, Caltrans estimated it would cost $1.3 billion dollars to replace the eastern span of the Bay Bridge. Five years ago, the state said it would in fact cost almost twice that much -- $2.5 billion -- for a new bridge.

Last year, Caltrans had still another projected cost: almost $5.3 billion dollars.

Wednesday night, Caltrans spokesman Jeff Weiss acknowledged the agency has had problems with estimates.

"Well, we've had a difficult time with this project, there's no secret about it," said Weiss.

The difficulties could grow even worse in the next month.

KTVU Channel 2 News has learned that just the soaring illuminated tower of the new Bay Bridge project may now cost a staggering 400 million dollars more than Caltrans originally thought.

In fact, the federal highway administration has presented Caltrans with working numbers that say constructing the signature 525-foot suspension tower may come in at $1.8 instead of the current official estimate of $1.4 billion.

In Sacramento, State Senate President Don Perata vowed the bridge will be built, but he remains skeptical of Caltrans.

Senator Perata explained: "I haven't heard those two words very often used together in the same sentence: Trust Caltrans."

Perata led the charge in 2004 against Governor Schwarzenegger over the cost of the bridge, a fight that led to a stall in construction.

The governor argued a different bridge design would save taxpayers money. Instead, Caltrans now says that interruption will probably cost taxpayers about $133 million more. And that doesn't even include the illuminated tower portion of the project.

Officials from the Federal Highway Administration declined to go on camera for an interview, but they did explain more details about their ongoing study.

It's important to note the new cost estimates are just that -- estimates.

If just one organization makes a bid for the contract, as happened the first time, the bid will likely come in high. If it turns out, there are multiple bidders, the cost could actually drop as much as $200 million.

Caltrans spokesman Weiss is optimistic: "Odds are we're going to be on budget with this project."

To give this some context, consider that the entire cost of the complicated earthquake retrofit project -- where the Bay Bridge touches down in San Francisco -- is costing taxpayers S200 million. That's just half of the latest possible cost increase.

No one will know exactly whether the federal study is correct until next month when Caltrans opens the bids for the new work on February 1st. Until then, taxpayers will just have to do what Caltrans has been doing all along: hold their breath.