Those factors are particularly evident in the breakdown of recordkeeping fees. In 2014, the median recordkeeping fee was $70 per plan participant, down from $80 in 2013. 

The majority of recordkeeping fees are determined by the assets in plan. That fees have fallen as assets have increased – the S&P 500 Index gained 32 percent in that time – suggests sponsors expect no less and that providers are trying to respond accordingly. 

In 2006, the first year that the Boston-based investment consultant conducted its survey, fees averaged $118 per participant. 

Overall average plan expense ratios – a metric that differs from median fees because it accounts for the revenue-sharing arrangements most plans have with mutual funds – fell to .49 percent, or 49 cents for every $100 in plan assets. In 2013, the average expense ratio was .52 percent; it was .57 percent in 2006. 

Ross Bremen, a partner at NEPC and co-author of the 2014 report, said other significant trends emerged as the data on this survey began to come in. 

"The biggest plans are showing signs of moving to fixed, dollar-per-head fees, and a growing portion have no revenue-sharing arrangements with recordkeepers," Bremen said. 

That said, Bremen noted that the majority of fees are still set relative to a plan's assets, and typically are paid for with some form of revenue-sharing from the plan. 

Also read: DC record-keepers hope less is more

On the other hand, according to this year's study, 29 percent of sponsors have arranged a fixed-dollar-per-head fee structure with third-party recordkeepers, and a growing portion of those sponsors have dropped revenue-sharing structures from their plan design. 

The study suggests that a fixed-dollar per head structure is the most transparent, and that increasingly, larger sponsors are insisting on that approach as a way to address their fiduciary liability. 

While only 33 of the 113 plans surveyed set a fixed-dollar per head fee structure, their average plan assets are nearly $1.5 billion. 

That left 72 of the 113 plans surveyed still arranging fees with recordkeepers on a "bundled" or "fixed-basis point" structure, which means the fees can move in relation to a plan's expense ratio or a plan's total assets. Those plans tend to have about $355 million in assets. 

And while only 16 of the plans surveyed have done away with all revenue-sharing arrangements from their plan's assets, 10 of them are plans with more than $1 billion in assets, suggesting that larger plan sponsors are moving away from the traditional, and potentially less-transparent and most costly way of compensating recordkeepers. 

"Larger sponsors have greater negotiating power with their service providers," explained Bremen. "They tend to be the ones that pace change for midsized and smaller plans." 

He said it remains to be seen how the trend of setting fixed, dollar-per-head fees plays out with midsized and smaller plans, but that with larger plans, sponsors are sending a clear message to recordkeepers: keep costs down.