President Carter yesterday signed an $11 billion farm bill that he described as "a good investment" although it is more expensive than he had in mind.

At a Rose Garden ceremony, Carter labeled the new law as most far-reaching farm legislation enacted in 40 years. It boosts grain farmers' incomes, revamps the federal food stamp program and expands agricultural research.

While the bill's food stamp and price support sections will be costly to taxpayers, specialists say the measure's immediate effect on prices at the supermarket will be minimal.

Complimenting members of Congress who fashioned the farm law, Carter said that "as a farmer myself." he was pleased to sign the measure that renews and revises nearly every statute governing Agriculture Department programs.

Carter's signature marked the end of almost three years' work by Congress and two administrations to prepare the bill, which will be in effect for the next four years.

The President hailed the law as "a great boon not only for American farmer families but for anyone who consumes our products."

The bill tightens eligibility requirements for families with incomes above the official poverty level, while increasing aid to families with incomes under the poverty level.

The $11 billion estimated annual price tag on the bill is about $2 billion more than Carter said he wanted to spend, but he said the bill will lead toward stability for the farm economy and farm programs.

Price support sections of the bill, in which the government guarantees minimum prices farmers can get for their crops, will cost up to $4.4 billion a year and the food stamp program will cost $5.6 billion annually, the administration says.

Actual spending on crop-related programs depends primarily on the weather in this country and in other major crop-exporting nations. Favorable growing weather, bumper harvests and slow foreign demand for U.S. grains would keep prices low, prompting higher government payments to help farm income keep pace with production costs.

The farm bill will provide a record $1.2 billion in such payments for wheat farmers, who have harvested a surplus-building crop this year of 2.03 billion bushels.