The Retirement Living II series saw a 24 basis point reduction in 2015, which lowered the average fees in that series from 76 to 53 basis points. The latest announcement of another nine basis point reduction across the Retirement Living portfolios brings the average fee to 44 basis points.

Fees on the Retirement Living Through series of funds remained even between 2014 and 2015, at 72 basis points, according to Morningstar. The nine basis point reduction put the average fees in that lineup at 63 basis points.

As in the universe of mutual funds, many target-date fund series continue to see steady declines in costs, as fund companies compete for the lucrative $760 billion-plus target date market.

More than half of the average weighted expense ratios in target date funds tracked by Morningstar declined in 2015, adding to a "multiyear" trend, according to the 2016 TDF Landscape report.

Staying competitive

From 2014 to 2015, the average weighted expense ratio across all target-date funds tracked by Morningstar fell from 78 basis points to 73 basis points.

"The decline is attributable to investors seeking out low-cost options and investment managers lowering fees to stay competitive," according to Morningstar's report. The average expense ratio for all funds was 103 basis points in 2009.

Lately, no target-date fund provider has been more aggressive in cutting fees than John Hancock. In 2015, 10 of the 54 series tracked by Morningstar reduced fees by at least 5 basis points. The 24 basis point reduction to the John Hancock Retirement Living II series and the 22-point reduction to the Retirement Choices series represented the largest price declines, with State Farm's LifePath series also reducing fees by 22 basis points.

Eight target-date fund series raised prices in 2015, with AllianceBernstein's Multi-Manager Select series increasing costs by 24 basis points, to 107 basis points, according to Morningstar.

The cost of target date funds is significantly impacted by investment style — some target-date funds are built exclusively on passively managed funds — and share class.

In 2015, Fidelity, T. Rowe Price, and Vanguard each launched target-date fund series aimed exclusively at the institutional market.

Fidelity's Freedom Index fund can be priced as low as 10 basis points, and while the W share class of that fund has no investment minimum, that price point is generally available only to 401(k) and other employer-sponsored savings plans that use Fidelity's record-keeping platform, according to Morningstar.

Vanguard's Institutional Target Retirement Fund series is also offered at an average of 10 basis points, but a $100 million minimum investment is required. Vanguard's record-kept clients can access the fund without that minimum, says Morningstar.

In 2015, target date funds attracted $69 billion of net inflows, an all-time high. Analysts at Cerulli Associates have predicted that target-date fund assets will hit $1.7 trillion by 2018, and will account for 90 percent of defined contribution assets.

According to analysts at Morningstar, target-date funds "have a clear runway for continued, steady growth serving as the default investment in many defined contribution plans."