While the pause in the drop of total costs may suggest a bottom has been found, Ross Bremen, head of NEPC's defined contribution practice, expects further declines next year.

"This projection is based on a few different factors, including sponsors who have been considering share class and contracting changes but have not yet made them and significant numbers of vendor searches in progress that have not been captured," Bremen said in a release.

To be sure, the declines in the cost of administering and investing in workplace plans over the past decade are substantial.

In 2013, the average per-participant record-keeping fee was $80. And in 2006, when NEPC first conducted its annual plan study, the average was $118. The study says 83 percent of sponsors have re-negotiated record-keeping fees since 2014.

Bremen thinks there is some risk that comes with the dramatic decrease in plan administration.

"Low fees have been a source of mixed emotions. While sponsors are able to highlight their good work by reducing fees for participants, it's done at the risk of hindering innovation and service. The race to the bottom is often a double-edged sword," said Bremen.

More than half of the plans reviewed have a per-participant record-keeping fee structure in place.

And 18 percent of plans pay a fixed percent of plan assets to record-keepers.

That practice has come under scrutiny in class-action lawsuits brought by plan participants, who allege fee-based charges to be imprudent under the Employee Retirement Income Security Act. The cost of record-keeping plans does not go up as plan assets grow over time, allege the lawsuits. With a fixed fee, service providers are able to earn more as plans grow without adding additional services, claim the plaintiffs.

Many larger plans with more than $1 billion in assets are moving to per-participant record-keeping arrangements—29 plans in 2017. NEPC's study is data-driven, and provides no speculation as to whether that trend represents an effort to limit fiduciary liability.

But it was the smallest plan segment, by asset size, that saw the greatest migration to per-participant record-keeping arrangements—19 plans with less than $250 million in assets moved to a per-participant structure in 2017.

Revenue sharing agreements—another target of plan participants in 401(k) lawsuits—are still common. NEPC's study shows 70 percent of plans use revenue from plan investments to cover the cost of record-keeping.

About a quarter of plans return revenue sharing payments directly to participants' accounts, while 60 percent of plans use the revenue to cover record-keeping costs.

In 2017, 28 plans eliminated revenue-sharing arrangements.

Larger plans have the benefit of accessing lower-cost investments from fund companies.

But NEPC's data shows the spread between the median plan expense ratio of the smallest and largest plans is relatively narrow.

Plans with less than 1,000 participants paid 48 basis points in 2017, a slight increase from the previous year.

The largest plans, with more than 15,000 participants, paid 33 basis points, a slight decrease from the previous year.