The cost of adding a new pair of tracks under the Hudson is now estimated at $8.7 billion.
The Port Authority is putting up $3 billion, and Washington will put up another $3 billion.
But Christie says he may have to consider other options instead of the tunnel.
On Friday, Democratic leaders assailed Christie for halting work on the tunnel and accused him of playing politics with a project whose costs he has been aware of for months.
"To miss this opportunity would be a huge mistake," Sen. Robert Menendez said as he stood in front a pile of dirt near the spot where the new tunnel would pass underground toward the Hudson River. "This is shortsighted in terms of the state's economy as well as the region's economy.
"I don't think it's time to waver, it's time to commit," Menendez added.
For its part, New Jersey has committed $2.7 billion to the project. Christie stopped work because of concerns that the project would go over its budget and New Jersey would be forced to cover the overrun, which some reports estimated could be as high as $1 billion.
Work that already has started in North Bergen and in a separate tunnel segment will be allowed to continue during the suspension.
But all new work, including land purchases and contract awards, will be suspended.
Through a spokesman, Christie defended the work suspension as a necessary move to take a close look at costs.
"It is reasonable and timely for us at this stage to undertake a careful review of the project, its costs and potential contingency requirements," the governor said. "That is what we are doing with the Federal Transit Administration during the 30-day period."
The tunnel, scheduled to be completed in 2018, is the largest federal transportation project in the country and expected to double train traffic in and out of New York during peak commute times, from 23 to 48 trains. Officials have estimated it will create 6,000 construction jobs and add at least 40,000 new jobs after it is completed.
In addition to New Jersey's part, the federal government and the Port Authority of New York and New Jersey have committed $3 billion apiece to the project. So far, $600 million has been spent, according to the lawmakers assembled Friday.
But Democratic lawmakers cautioned the state is in jeopardy of losing the full $3 billion committed by the government, which federal transportation officials said is the largest grant of its kind ever given.
"Every other state is waiting to get their mitts on our hard-earned money," U.S. Sen. Frank Lautenberg said. "We don't want to be just another parking lot for New York City, and that's what'll happen if we don't get this tunnel under way."
Lautenberg, Menendez and others said Christie demonstrated in correspondence with Transportation Secretary Ray LaHood earlier this year that he was aware of the $8.7 billion price tag and the fact that New Jersey would be liable for cost overruns.
State legislators were to meet Monday to address the work suspension. Democratic state Sen. Raymond Lesniak has suggested a $2 surcharge on commuter fares and forming public-private partnerships to raise money to ensure the tunnel is built.