The Jets said it would cost $370 million but now say it will run about 10% less. The Madison Square Garden people said they could do it for $250 million. The Regional Plan Association estimated that for its mixed-use “urban design alternative,” the price tag would be $388 million.

And some observers said it should cost much, much less.

The projected cost to build a platform over the Metropolitan Transportation Authority’s rail yards between West 30th and 33rd streets varies enough to make your head spin. Even as a moving target, however, it has been one of the few tangible numbers offered up in the politically charged battle over the future of a valuable swath of land by the Hudson River.

So how much will the platform really cost? Representatives of some of the leading local engineering firms declined to speak on the record, for fear of alienating the powerful players involved in the battle. Most agreed the costs could indeed vary widely, depending on many factors, ranging from how willing the MTA is to shut down Long Island Railroad tracks during construction, to the height and weight of the structures that developers plan to construct above the train yards.

Mayor Bloomberg has made a top priority of building a state-of-the-art facility featuring a 75,000-seat domed stadium, which his team has dubbed the New York Sports and Convention Center.

Housing activists and Cablevision, which owns Madison Square Garden, would prefer to see the land used for residential and commercial development and have offered counterproposals. The site in question is the western portion of the MTA’s rail yards between 30th and 33rd streets and 11th and 12th avenues. In addition, the city wants to develop the eastern rail yards, an area of similar size that runs between 10th and 11th avenues, for housing and office buildings.

The “platform” refers to the structure that will support whatever is built over the western rail yard and the foundations that will anchor it in the ground.

The MTA-operated rail yard has approximately 30 rows of parallel tracks that snake across the property into Penn Station. Whoever constructs the platform will have to do so while the rail yard continues to operate.

Under a plan negotiated by the Jets and the Bloomberg and Pataki administrations, the city and state governments would each contribute $300 million to finance the platform and the stadium’s retractable dome and air conditioning. The Jets would pay the estimated $800 million to build the stadium.

The Madison Square Garden offer of $600 million for the development rights, which precipitated the MTA’s announcement Tuesday of an open bidding process, was to be reduced by the cost of erecting the platform.

Here’s what the different development plans agree on. The foundation will probably consist of a number of piles and caissons – basically, large hollow tubes filled with concrete – drilled into the ground into areas spaced about 50 feet apart between the tracks. A “lateral grid” of beams and trusses will run between the piles and caissons, distributing the weight from above evenly. In some portions of the yard, the ground is soft landfill, so engineers will need to dig as deep as 100 feet to hit sold rock below the earth.

That’s probably where the agreement ends.

The New York Jets have spent the most time working up costs of the platform, consulting for the past three years with Turner Construction and Tishman Construction to come up with estimates. Project planners have met with officials from the LIRR about every two weeks to coordinate for more than a year and a half, according to Thad Sheely, vice president of development for the Jets.

He said that under normal circumstances, it would take about nine months to construct the platform that would support the stadium. Since the LIRR has indicated a willingness to take out between two and four tracks at a time, the project can probably be done in 30 months. He said the platform would be cheaper to build for the stadium than for residential or office towers.

“Our foundation work will be less expensive because the stadium load is evenly spaced and it can be done all at one time,” Mr. Sheely said.

The Jets came in with initial estimate of $370 million for the platform, but Mr. Sheely said they went back through and managed to bring the number down to $335 million by increasing the strength of some of the caissons, so that fewer of them would be needed to support the foundation.

The New York City Economic Development Corp. has been working on the project for two-and-half years, with a team of architects from Turner, in connection with developing the eastern portion of the rail yards for residential and commercial housing.

Cost estimates calculated by Turner, which city officials have reviewed, amount to about $350 million if developers of the western portion were to erect a platform of 570,000 square feet for apartment buildings, according to the EDC.

The Regional Plan Association, which has opposed the stadium plan, has used the city’s numbers for the eastern part of the rail yards to estimate how much it would cost to develop the western side for residential and commercial uses instead of as a stadium, according to a spokesman for the nonprofit civic group, Jeremy Soffin. Association experts, who Mr. Soffin said consulted engineers off the record, estimated the platform for the western part could be built for $388 million.

“Our estimate was considered an extremely conservative figure,” he said. “Their sense was that it could be done for half that cost. The feeling was it could be done much cheaper.”

Cablevision’s Madison Square Garden, when it made its competing offer, placed the cost of the platform at $250 million, producing a net offer for the development rights of $350 million. Its officials acknowledged, however, that the estimate was preliminary. If the platform costs more, they said, they would be willing to negotiate additional costs with the MTA. The preliminary estimate was based on consultation with Severund Structural Engineers and Bovis Land Lease.

MSG officials declined to comment on the record.

Whether the actual cost to build a platform will actually conform to those estimates remain an open question.

The former MTA chairman who is credited with having built the rail yards, Richard Ravitch, called current talk about the platform “disingenuous nonsense.” He said the platform would cost far less than the city and the Jets were letting on.

Mr. Ravitch, a longtime public servant whose private career has been principally as a builder, said it was actually the steel supports that run between the caissons to hold up the stadium that would constitute the bulk of the costs.

“What’s going to support the stadium is a massive truss, and my guess is the city included the truss in the platform costs to mask the cost of the subsidy to the Jets,” he said. “There’s nothing wrong with that, but it raises a number of public policy and financial issues.”

Jets officials acknowledged that the cost of the steel that runs between the caissons would probably cost one-and-a-half times as much as the actual caissons and was included in their platform estimates. But they said building a residential or commercial development would require just as much steel.

And they said it was MSG whose estimate is disingenuous.

“Our message is that MSG has come in here with a sham bid which doesn’t really take into account the cost or complexity of building a platform over the rail yards in this site,” Mr. Sheely said.

Mr. Sheely said the MSG plan would cost more than $250 million because the foundation would have to be more complicated. The weight of the stadium, in contrast, would be equally spread out over the area of the rail yard, he said.

Officials at the Economic Development Corp., which consulted the same engineers as the Jets and through its connection to the Bloomberg administration supports the stadium, made the same point.

Residential towers might actually have some buildings that are 600 feet high, while others could be one story, vastly complicating the cost estimates for the platform on the western side, EDC officials said.

“Using the same engineers and cost estimates as the Jets, the city estimates that a platform for mixed-use development, including 1 million square feet of residential, would cost $351 million,” an assistant vice president at the EDC, Michael Meola, said.

MSG officials acknowledged that its buildings in some areas will cost more to support than a stadium but said that in other areas their platform would cost less. They said they were conducting additional studies on the site and the cost to develop it.