When a discount broker offers to save its customers "up to" 70 percent, 80 percent or even 90 percent on commissions, the wise investor should question that claim, the Better Business Bureau of Metropolitan New York has warned.

During a three-month study of advertising and promotional literature used by discount brokers, the BBB said it found a wide variety of claims and practices that may confuse and mislead investors.

The BBB, which called for full disclosure by the discounters, said that the advertising of 19 out of the 24 discount firms it studied failed to state clearly:

*How the discounters arrive at their claims of large percentage savings.

*How big a transaction is needed to obtain the maximum savings.

*How many clients receive the maximum discount.

*The minimum savings a customer can expect.

BBB President Barbara Berger Opotowsky said the group undertook the survey because of the growing number of consumers being affected by the proliferation of financial services.

Brokerage operations "are not an elite kind of thing anymore," she said. Opotowsky said the BBB found that current discount-brokerage advertising "does have the capacity to mislead."

She added: "It is unfortunate that the credibility of this particular type of advertising leaves something to be desired."

The National Association of Securities Dealers, which operates the over-the-counter market, said it agreed to review the findings of the BBB investigation relating to member firms.

"To the extent we find they are violative, we will tell the firm to stop using the ads, and if they don't comply we would institute disciplinary action," said Frank J. Wilson, NASD executive vice president.

A New York Stock Exchange spokesman said the NYSE had not received a formal request from the BBB to study the bureau's findings.

The NASD and the NYSE both conduct routine spot checks of member firms' advertising literature. Because of this ongoing review, some firms said they feel they are complying with industry standards. They also noted that new customers are given full information on all rates and fees.

Discounters generally reacted to the BBB report negatively, and denied they were attempting to mislead customers.

"I really think it's a mountain out of a molehill," said Leslie C. Quick III, vice president of Quick & Reilly, one of the nation's largest discounters.

However, as a result of the report, Quick said, his firm's advertisements will again include a reference to its $35 minimum fee. Quick said his firm discontinued the practice several years ago with the approval of the NYSE. Of the 25 discounters reviewed, only Quick & Reilly and four others changed their advertising practices to conform to the BBB's suggestions.

Quick & Reilly and several other discounters said their claims of 70 to 90 percent off were based on comparisons with the old fixed commission rates abolished by the Securities and Exchange Commission in May 1975. The end of fixed commissions opened the way for discounters to enter the market. Discounters will execute a customer's order, but do not offer investment advice and do not have research departments.

Typically, the listed, but negotiable, rates of a full-service house, Prudential-Bache, exceed those of discounter Charles Schwab & Co. by between 51.7 percent and 237.5 percent on trades involving a $30 stock. A 100-share trade costs $74.35 at Prudential-Bache and $49 at Schwab. A 1,000-share trade is $486 at Prudential-Bache and $144 at Schwab. Discounters, the BBB complained, use so many methods to arrive at their "70 percent off" figures that there is no way for the investor to know what the comparisons mean. Firms may compare their commissions to the pre-1975 fixed rates, the current full-service rates, or even to the rates used by other discounters.

Sometimes out-of-date information is printed, and often none is given, the BBB said. Without disclosure, the BBB said, "comparative savings claims become virtually meaningless."

Maurice Minerbi, manager of the discount brokerage operation for T. Rowe Price Investment Services of Baltimore, said his firm's claims of "up to 75 percent" off were based on 1983 figures for "typical full-service brokerages."

He said his firm's ads were in line with industry practices, and that the BBB was "making their own set of rules in the middle of the game." He said he would not object to including the date and source of the commission comparison "if everyone were required to do so."

In response to listing minimum savings, Minerbi said the minimum could be zero because the T. Rowe Price minimum commission of $30 might be more than that charged by a full-service broker. Less than 15 percent of T. Rowe Price customers get the maximum discount because of the size of the trades required, he said. As an example, he cited a trade of 2,000 shares at $5 a share, or $10,000, as qualifying for the full 75 percent off.

Muriel Siebert, the first woman to hold a seat on the New York Stock Exchange, said her firm, Muriel Siebert & Co., advertised savings of "up to 90 percent" based on current full-service rates, not the 1975 rates.

She took the same stand as Minerbi on minimum savings, saying, "It is not practicable or possible to define minimum savings on a percentage basis."

The BBB said it also wants discounters to disclose how large a trade has to be for a client to get the maximum savings. In some cases, the bureau said, the maximum discount can be achieved only with a trade of $50,000 or more. Some discounters declined to include that information in advertisements, saying maximum savings can occur in a variety of circumstances, the bureau said. Firms also declined to specify how many customers received the highest savings, the BBB said. The BBB said that it raised questions about advertising practices of 19 companies: Baker & Co. Inc.; Brown & Co. Securities Corp. Inc.; Bull & Bear Securities Inc.; W. T. Cabe Inc.; Discount Brokerage Corp. of America; Fidelity Brokerage Services Inc.; Marsh Block & Co. Inc.; Olde & Co. Inc.; Ovest Financial Services Inc.; T. Rowe Price Investment Services Inc.; Quick & Reilly Inc.; Rose & Co. Investment Brokers Inc.; Charles Schwab & Co. Inc.; Seaport Securities Corp.; Security Pacific Brokers Inc.; Muriel Siebert & Co Inc.; Spear Securities Inc.; Vanguard Discount Brokerage Services; and Waterhouse Securities Inc.

The BBB was satisfied with the advertising practices of five companies: Haas Securities Corp.; Pacific Brokerage Services; Andrew Peck Associates Inc.; Tradex Brokerage Service Inc.; and Wall Street Discount Corp.