Because collective trusts are not registered with the Securities and Exchange Commission—the Office of Comptroller and Currency, and in some cases, state banking regulators oversee CITs—they carry fewer disclosure requirements, which cost money.

And because they are only offered to participants in qualified workplace plans, and not retail investors, fewer marketing costs are baked into trusts' expense ratios.

Just how cheap are TDFs in CIT form?

Morningstar's data, which is self-reported from industry, is perhaps the most comprehensive.

Of the more than 1,500 listed trusts—different target-date years of the same trust family are broken out in that list—the average expense ratio is about 60 basis points.

For target-date mutual funds, the average net expense ratio of institutional class shares is 0.918, as reported in fund prospectuses, according to a senior research analyst at Morningstar.

In the CIT target-date universe, variances between management style—pure passive approaches exist, and create even greater cost efficiencies—and different institutional share classes make for a fairly wide spread between the least and most expensive target-date trusts tracked by Morningstar.

How cheap can they get?

The BNY Mellon SmartPath target-date 2055 fund lists at 0.003, or one-third of one basis point. According to data reported to Morningstar, the fund holds about $1.2 billion in assets.

Smaller funds, issued by regional trusts, come with price tags more normal to the fund world. Hand Benefits and Trust, the trust company of Benefit Plan Administrative Services, sponsors the First Trust target-date 2010 trust, for about 150 basis points.

Jake Gilliam, Senior Multi-Asset Class Portfolio Strategist at Charles Schwab Investment Management, broke out the discrepancy in share costs of the firm's two suites of target-date trusts, each of which has an open-architecture design—which is to say no proprietary Schwab investment products are built into the funds.

Schwab has been preaching open architecture since it started building TDF trusts in 2002, says Gilliam.

"We design these products for the fiduciary buyer," said Gilliam. "They come with a very low cost, are fully diversified, and only use open architecture. We think there are very few open architecture products that can compete with us."

The most inexpensive expense ratio is eight basis points, available in Schwab's indexed trust line to sponsors willing to map at least $100 million into the fund. Without the mapping, the expense ratio is 14 basis points.

Schwab's other series, the SMRT managed target-date trusts, offers five different share classes, with expense ratios ranging from 35 to 89 basis points—to get the best value, sponsors must be willing to map a minimum of $300 million into the fund.

In the eyes of the Employee Retirement Income Security Act, cheap is not synonymous with best. The law often criticized for being opaque is clear in one way: Plan sponsors can't duck fiduciary obligations simply by stacking lineups with the most inexpensive funds.

Performance matters too. Each fund within Schwab's series of actively managed target-date trusts ranked in the top quartile of its peers in 2015, according to a statement from the company.

Two of its shorter-dated funds ranked number one.

Gilliam says Schwab's conservative approach to glide-path design and scrutiny of third-party managers benefits the near-and-newly-retired investor, particularly given recent volatile market conditions.

About 40 percent of Schwab's trust portfolios are invested in equities as investors approach the culmination of glide paths. Schwab further hedges against risk by removing more volatile commodity and emerging market investments from the equity allocation as retirement nears.

And fixed-income takes on more Treasury Inflation Protected Securities, said Gilliam.

Combined with that level of risk management, the funds' open-architecture design distinguishes Schwab's trust offerings from its own line of TDF funds, which do incorporate Schwab proprietary products.

"Open architecture is a strong differentiator," said Gilliam. "It allows us to change managers easily, if needed, and that makes sponsors' jobs easier as fiduciaries. The alternative is to remove an entire investment option if their are issues with the management of a proprietary fund."

That can be cumbersome, for sponsors and participants, says Gilliam. He points to a Department of Labor fact sheet on TDFs, issued in early 2013, that suggests sponsors consider non-proprietary products, because they diversify participant exposure by incorporating strategies beyond the provider's.

Gilliam argues that open architecture in target-date trusts adds another layer of fiduciary value for sponsors.

Last year, Schwab's CIT target-date lines clocked 14 percent organic growth, including several large-plan deals.

"Over the last few years, sponsors and plan consultants have really started digging into TDF design," said Gilliam. "Yes, fund design has evolved, but so has sponsors' understanding of the products."

He expects that to continue. By the end of 2013, assets in all CITs (not just TDF trusts) were pushing towards $2 trillion. The Coalition of CITs—an advocacy formed four years ago—says 60 percent of sponsors were offering trusts as an option in investment menus by 2014.

So if the trusts are cheaper, and the market incorporates the panoply of glide path and management design offered in the TDF fund market, the question remains: Why even consider not using the more affordable trust option?

"For some sponsors it's a question of comfort level," explained Gilliam. "Others want choice. And we want them to have that choice."

He added, "We love that they're looking for more than one product."