CHICAGO (

TheStreet

) -- The U.S. Supreme Court on Monday heard arguments in Jones v. Harris Associates to decide what constitutes excessive investment advisory fees in the context of the fiduciary duties expressed in the Investment Company Act of 1970.

For non-lawyers among us, shareholders are asking whether it's OK for investment advisers to charge different shareholders higher fees as long as they're disclosed transparently.

Both sides have compelling arguments.

The attorney for Jones, who represents three investors in

Harris Associates'

TheStreet Recommends

Oakmark Funds, said investment advisers do their research and execute block trades. The shares are allocated to various fund classes, yet the adviser charges some classes more than twice as much as others for the same service.

How widespread is that practice? TheStreet.com data show that a third, or 1,164, of the 3,391 open-end stock mutual funds with multiple share classes charge investor-share classes double or more than institutional-class shares of the same fund.

Lawyers for Harris Associates claim that transparency and competition ensure fairness, and investors in Oakmark Funds are charged only a small expense ratio while being well-compensated by strong fund performance.

Ira Schochet, president of the National Association of Shareholder and Consumer Attorneys, called Wall Street advisory fees "frequently excessive" and said they "can seriously erode fund investors' returns."

Below are funds with more than $100 million of net assets in both share classes with the widest difference between expense ratios. In absolute percentage terms, those funds don't necessarily have excessively high expense ratios. However, when you compare the charges paid by favored customers for the same product, the expenses are significantly higher on a relative basis, as alleged in Jones v. Harris Associates.

As half of American households have upwards of $9 trillion invested in mutual funds, the Supreme Court decision could open the door to lawsuits from as many as 92 million shareholders that could drive down, or at least level, fees.

Kevin Baker became the senior financial analyst for TSC Ratings upon the August 2006 acquisition of Weiss Ratings by TheStreet.com, covering mutual funds. He joined the Weiss Group in 1997 as a banking and brokerage analyst. In 1999, he created the Weiss Group's first ratings to gauge the level of risk in U.S. equities. Baker received a B.S. degree in management from Rensselaer Polytechnic Institute and an M.B.A. with a finance specialization from Nova Southeastern University.