A hotly debated plan to make Mondays a full day for elementary students in Fairfax County may cost nearly 30 percent more than Superintendent Robert R. Spillane has projected.

According to estimates produced by school officials last summer, eliminating the Monday early closing would cost $7.2 million, not the $5.6 million the superintendent estimated in his formal proposal last month.

Critics of the plan said the discrepancy shows that Spillane has deliberately underestimated the cost to make his proposal seem financially palatable.

"Our past experience with the superintendent's proposals would lead us to conclude that what he submitted is a low-ball figure," said Maureen Daniels, president of the 6,800-member Fairfax Education Association. "It's really hard to pin down and believe what's being presented when so much is being misrepresented."

Spillane stood by his estimate, calling the previous ones working numbers.

His allies suggested opponents are grasping any ammunition they can find to sink a plan they don't like.

"I trust the staff work and I think the board needs to keep its eyes focused on the big picture: what this will do for 70,000 kids," said School Board Chairman Kohann H. Whitney (Centreville), the prime champion of the plan.

"The opponents are going to try to raise a lot of false issues and this is one of them."

The cost of the plan, which will be voted on next Thursday, has become a focal point at a time when flat fiscal forecasts have principals looking for ways to trim spending on textbooks and field trips.

Last month, a majority of the Board of Supervisors urged the School Board to table or reject the proposal because even at $5.6 million, they consider it unaffordable.

"There are no new initiatives that we can afford," said Board of Supervisors Chairman Audrey Moore (D).

"It doesn't matter whether it's a third more {expensive} or not; the money is not there," she said.

Fairfax began closing elementary schools as much as 2 1/2 hours early on Mondays 20 years ago to give teachers time for planning, staff development and parent conferences.

Last year, the School Board voted in concept to end that practice, but delayed consideration of a specific plan until this fall.

The different cost estimates of the plan arise from differing estimates on the number of teachers who would have to be hired.

To make up for the planning time teachers would lose on Monday afternoons, Spillane proposed hiring 129 new teachers to relieve homeroom teachers of their physical education and art responsibilities.

But in a document prepared for a community task force studying the issue last summer, Spillane's staff estimated that such moves would require hiring 175 teachers.

"I haven't gotten a satisfactory explanation for the difference," said School Board member Letty A. Fleetwood (Providence).

"I just don't think that this is the total figure. I don't think even the $7.2 {million} figure is."

She and other critics noted that class sizes have increased dramatically in some high schools this year with the addition of a seventh class period, proof to them that the plan last year was inadequately funded.

Rachel Verville, the school official who headed last summer's Monday task force, said the earlier numbers were rough estimates.

"We worked with ballpark figures because it takes so much time to cost out exact numbers for each scenario," she said.

Spillane, Whitney and others also maintain that, in an operating budget that likely will exceed $900 million next year, the Monday plan is a bargain at either price.

The elementary day plan has taken a tortuous path through financial planning from the beginning.

When some School Board members wanted to extend Mondays about two years ago, school staff members estimated it would cost about $24 million, which quickly put an end to the idea.

Last year after Spillane and the School Board decided to extend Mondays once and for all, his staff produced a new estimate of $8.8 million.

At the time, school officials explained that the 1988 estimate had been too high because they overestimated increases in bus transportation.

Last year's estimate was based on the prospect of hiring 220 new teachers, but that plan was more ambitious in giving students more time in specialized subjects.

Spillane denied that he was fudging numbers for political reasons. "I wouldn't do that," he said. "I try to be realistic."