A monopoly among the nation's major producers of ready-to-eat cereals cost the nation's consumers more than $1.2 billion in higher grocery store prices between 1958 and 1972, overcharges that add 15 percent to every dollar spent for the cereal products, the staff of the Federal Trade Commission charged yesterday.

Release of the new data and the charges about alleged industry monopoly practices is the final commission staff volley in the eight-year legal battle surrounding the FTC's controversial cereals case.

The FTC staff also said yesterday that the companies that are both the targets of a landmark FTC antitrust case and the three major players in the industry -- Kellogg Co., General Mills Inc. and General Foods Corp. -- compiled overcharges of more than $420 million for the five-year period ending with 1970 and are responsible for about $1 billion of the 15-year excessive consumer prices.

For 1971 and 1972, the agency's staff said that "monopoly profits" for Kellogg's and General Mills were as high as $38.5 million and $35.8 million, respectively, and that for the period from 1966 to 1970 the three respondents in the case overcharged consumers an average of $86 million a year.

The commission staff, in a 769-page summation of its antitrust case against the three major cereal manufacturers, charged that they have supressed competition through a "tacit conspiracy" involving the sharing of current advertising data and limitations on the use of discounts to grocery retailers and wholesalers.

In addition, the commission's trial staff said the companies adopted other restrictions on competition such as limitations on shelf space and an "understanding" on common responses to criticism of the lack of nutritious value of the cereal products.

The statistical evaluation of the magnitude of the alleged industry conspiracy is particularly unique and appears to represent the first time the federal government has attempted to tabulate the prices consumers have paid as a result of monopoly practices throughout one industry.

The complaint against the cereal companies was issued by the commission nearly eight and one-half years ago and was widely viewed as a precedent-setting case. It was the government's first major effort to attack a "shared monopoly," or a case in which it was charged that several companies dominated a particular industry.

Quaker Oats Co. was part of the original complaint, but the case against Quaker was dismissed on Feb. 24, 1978.

The industry, with retail sales of more than $740 million, is believed widely to be among the nation's most highly concentrated -- and profitable -- industries. The FTC staff said that the three respondents in the case controlled more than 80 percent of the industry's output. The industry's rate of return from 1952 to 1970 was nearly 20 percent compared with almost 9 percent for the nation's entire manufacturing sector.

Trial of the case began in April 1976 and, with a series of interuptions, was not concluded until Feb. 14, 1980. But the case also has been the subject of considerable procedural controversy over the retirement and rehiring of an administrative law judge, Harry Hinkes, who was assigned to the case.

Kellogg, in particular, has made a major political issue of the retirement and subsequent rehiring of Hinkes, who was given a contract for $72,000 -- or $23,000 more than his previous yearly salary -- by FTC Chairman Michael Pertschuk to continue the case. Known as the "Hinkes affair," the contract flap also has been the target of continuing congressional investigation.

Nevertheless, the case has gone on and, in light of the staff filing yesterday, the cereal manufacturers have four months to submit their legal briefs. After that the new law judge hearing the case, Alvin Berman, has six more months to file his opinion before the case goes to the commission for decision, presumably in about a year.

In seeking divestiture and other relief in the case, the FTC staff proposed that Kellogg be split into three other separate companies, and General Foods and General Mills each be split into one other firm.

In addition, the FTC staff proposed that the firms be forced over the next 20 years to lift trademark restrictions which prohibit grocers, for example, from using the manufacturer's brand name on house brands. The staff also wants to bar the cereal firms from acquiring competing companies and setting shelf-space rules.

The calculations on the impact of the alleged monopoly practices take into account ratios of cost to price, what the staff called "extraordinarily high" and "wasteful" advertising levels, and other costs -- "such as the cost of new-product introductions, competitive monitoring and others -- that are imposed as a result of the existence of monopoly power."