Image: 401k © Jan Cobb Photography Ltd., Photographer's Choice, Getty Images

America's conversion to the 401k plan has certainly created a new world for future retirees, who have been discovering just how tricky it is to prepare for retirement in their spare time. But one party hasn't been complaining: the financial-services industry.

Though most Americans know them primarily as brokers or banks, the Fidelitys, Merrill Lynches and INGs of the world also dominate the 401k packaging business. That allows those companies to introduce millions of workers -- and $4.3 trillion of their savings -- to the packagers' own hand-picked rosters of mutual funds. In total, financial firms take home somewhere around 1% of those assets a year, and few workers understand how that money flows. (One confusing concept: "revenue sharing." More on that in a bit.) But in coming months, the shroud of mystery may be lifted a bit, as the government is expected to implement regulations for disclosing fund fees.

Lori Lucas, a defined-contribution-plan specialist at consulting firm Callan Associates, says the new rules "will improve transparency and put additional pressure on plan vendors to make fees reasonable."

What investors see once that transparency arrives might be quite eye-opening. By rough estimates, 401k fees add up to anywhere from $30 billion to $60 billion a year. Do the math and that comes to as much as $164 million every day. The companies say they more than earn that impressive income stream, given the complexities of record-keeping and accounting for tens of millions of accounts -- not to mention investing the money. In testimony to regulators, fund companies have said their 401k charges compare favorably to the costs of getting the same services outside the plan.

There's just one problem: Most employees don't or can't comparison shop. In one recent study by AARP, seven in 10 workers said they didn't realize they were paying any 401k fees at all.

"There are enormous dollar amounts involved," says former plan consultant Frank Cirullo. "Employees are getting ripped off."

Indeed, even though advisers always stress the importance of keeping investing expenses down, many 401k plans don't do so. At a midsize company, annual fees can run less than 0.3% of assets for a no-frill plan that emphasizes inexpensive index funds. The fact that the average plan member pays three times that much suggests that cost efficiency isn't high on the agenda.

Oddly enough, employers don't always know what the bill for their 401k plan adds up to. When you ask them, says Glenn Jensen, managing director of New England Retirement Consultants, "you get a deer-in-the-headlights look."

Some of the problem, according to critics, comes down to that practice known as revenue sharing. In many plans, the packager charges the employer little or nothing. But the packager maintains leeway over which investment choices get included. And it makes up its costs by steering plan investors toward mutual funds -- usually more expensive, actively managed ones -- that route a slice of their fees back to the packager. On some funds, those revenue-sharing fees alone can cost an average of $33 a year per $10,000 invested, according to human resources consultants Aon Hewitt, enough to put a serious dent in an employee's investment performance.

Critics like Ryan Alfred, co-founder and president of financial research firm BrightScope, say this arrangement effectively discourages packagers from offering lower-cost index funds, because they can't pull in as many fees. The result: Plan members pay much higher fund-management fees, yet another factor that can eat into their returns.

Fidelity declined to discuss the relative costs of its plans, but says that in some cases, employers get to decide whether they want to use a revenue-sharing system. Bank of America Merrill Lynch and ING declined to discuss costs or revenue sharing, although -- irony alert -- both companies said they were in favor of transparency on fees. (Employers and workers "should fully understand costs and get real value," Merrill said in a statement.)

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Some consumer advocates are optimistic that the new fund-disclosure rules will untangle some of this mess and bring costs down. Still, problems are likely to persist. As currently proposed, the rules require packagers to disclose revenue-sharing fees to employers, but not directly to plan members, who will have to dig through regulatory filings to get them. And employees still lack straightforward ways to pressure their employers to cut costs.

Bottom line: The tussle over fees has only just begun.

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