PRESCHOOL has tremendous power. It inoculates children against later problems at school and work, and steers them toward lifelong success. Until now, however, the prospect of statewide, publicly funded preschool had a daunting price tag of $1.2 billion, making it seem like a lovely but unaffordable dream.

Now there's a new estimate of $600 million, or roughly $3,000 per child. The lower cost would require trade-offs in the state's universal preschool program, which is still in the planning stages. But it's a better price for a state that has other bills to pay.

Two factors helped lower the cost. First,an initial plan to offer four free hours of preschool has been scrapped. Some families would have to share the costs, as they do now. That's a loss. Ideally, preschool should be a free local resource, just like public school . But this compromise should help speed action.

The second factor was a recent survey that measured the demand for preschool and found that some parents wanted fewer hours than was initially anticipated.

"This is low-hanging fruit," says economics professor Paul Harrington, who points to the ease, high payoff, and marketplace efficiency of investing in 3- to 5-year-olds. Over time, children in quality preschool behave better, are less likely to drop out of school, and are more likely to become employed.

Harrington and his colleagues at the Northeastern University Center for Labor Market Studies came up with the new estimate in response to a legislative query. It's a step toward making the vision real, according to Margaret Blood of the Early Education for All Campaign, an advocacy group that commissioned the study.

Even at a lower price, the state could have high quality. The $600 million includes higher salaries for teachers with bachelor's degrees, because research shows that having a college-educated staff improves the quality of children's experiences. Harrington adds that more preschool jobs with higher salaries could keep people in the state by offering a career to older workers, the annual crop of underemployed college graduates, and parents who want to re-enter the workforce.

Statewide preschool could boost MCAS scores by giving children the early cognitive, social, and emotional skills they need to master English and math later on. Research shows that preschool most benefits children with the toughest educational challenges.

The estimate intentionally avoids specifics of how to spend the money. That's work for the Department of Early Education and Care and local providers. But armed with a realistic estimate, they can make progress.