The fees public pension plans pay Wall Street money managers -- some of which go unreported -- have come under increasing scrutiny in recent years. It's estimated that disclosed and undisclosed fees cost public plans upwards of $20 billion annually, according to the author of a new study.

That's a big dollar amount when you consider that public pension plans' collective unfunded liability is a little over $1 trillion. So far, just a few state plans have been trying to get a handle on these fees. One of them, the California Public Employees Retirement System (CalPERS), reported late last year that it paid $3.4 billion in undisclosed fees over the past 25 years on $24 billion in total investment earnings. CalPERS is the nation's largest retirement system.

Jeff Hooke, a consultant for the right-leaning Maryland Public Policy Institute, estimates in the study that Maryland's public employees' plan paid $500 million in 2014 -- twice as much as it reported for that year. Hooke said that if other states' hidden fees are similarly underreported, the total fees pensions actually pay could be as much as $20 billion annually. "And that's just for states -- forget about all the counties and cities," he said, "which could easily add another 25 percent to that."