That alone would mean up to $44 billion in investor gains over 10 years, the DOL says.

"These gains alone would far exceed the proposal's compliance costs," the DOL claims in its analysis.

How the DOL calculated its cost estimates, and their ultimate accuracy, are critical to the argument in defending the proposal's overall value-proposition to retirement investors.

In 2010, the DOL took criticism for not providing a robust enough analysis of what the proposal would require in compliance costs.

Since then, attempts to solicit data from industry to develop cost estimates have been ignored, says the DOL, forcing the agency to rely on data submitted to the Securities and Exchange Commission in 2013, as that agency set out to research its own cost analysis of a prospective uniform fiduciary standard.

The SEC had better luck engaging industry than did the DOL. The Securities Industry and Financial Markets Association provided data to the SEC from 17 of its broker-dealer members.

Ultimately, that data proved to be instrumental to the DOL's cost estimates. In fact, the data SIFMA provided the SEC, along with data from the Investor Advisor Association, an advocacy group for RIAs, and data from Charles Schwab, served as primary sources for the DOL's cost estimates.

SIFMA estimated average costs per firm in two areas. The first was the cost of developing and maintaining a disclosure form and customer relationship guide. SIFMA estimated those start-up costs between $1.2 million and $4.6 million per firm, with average annual ongoing costs of $631,000.

SIFMA also estimated average costs of implementing compliance oversight and training programs at $5 million to implement, and $2 million in average annual costs to maintain oversight, per firm.

While those estimates were relied on to build its own overall cost analysis, the DOL questioned the veracity of SIFMA's data.

"There is a reason to be concerned that the SIFMA submission significantly overestimates the costs of the new proposal," claimed to DOL in its cost estimate.

That statement doesn't sit well with Kevin Carroll, managing director and associate general counsel at SIFMA.

"The data we provided to the SEC had nothing whatsoever to do with the DOL proposal," Carroll said. "So I don't know how they can rightfully say we overstated the cost of our estimates."

He called the DOL's claim flatly inaccurate.

"They have no basis for saying that. It is a misuse of data," he added.

In basing its analysis on SIFMA data, the DOL conceded there "will be substantive differences between the Department's new proposal and exemptions and any future SEC regulation that would establish a uniform fiduciary standard for broker-dealers and RIAs."

Nevertheless, the DOL says SIFMA overstated the ongoing costs to comply with a new regulation by as much as 450 percent, basing that conclusion on data compared to the IAA's, which of course is based on RIA firms.

The DOL says RIAs would be expected to incur substantially lower costs to comply with the proposes regulations and exemptions."

For Carroll, it's the DOL's estimations, not SIFMA's, which fail to achieve accuracy, "The bottom line is that the DOL proposal would be another very large cost and burden on our industry."

Credit Suisse analysts of the brokerage industry agree. On top of the cost of implementing and maintaining compliance oversight, the DOL's proposal also increases the scope of enforcement action and the "ability for investors to sue for bad advice."

That is a "non-trivial risk," wrote a team of Credit Suisse analysts in a note to investors.

Pat DiCarlo, an Atlanta-based ERISA attorney with Alston and Bird, expects a new proposal will keep him busy "for years."

Though the DOL does estimate new increases to professional liability insurance premiums for broker-dealers, its assessment stops short of trying to factor in prospective costs of litigation DiCarlo says will be inevitable, and likely significant.

"It's a no-brainer," DiCarlo said. "The proposal provides an arena to sue that didn't exist before."

Specifically, he said the Best Interest Contract Exemption advisers would be required to provide IRA investors could give way to a significant expansion in class-action claims against broker-dealers.

"Like litigation in general, some claims will be more merited than others," DiCarlo said.

He doesn't fault the DOL for failing to incorporate potential litigation costs into its analysis.

"Even accurately estimating the costs to litigate one case is hard to do. Trying to do so for a whole new potential species of ERISA claims—that's virtually impossible," DiCarlo said.

Carroll said the broker-dealer members he represents at SIFMA are "scratching their heads," asking themselves, "can we even do this."

But one thing he said SIFMA members are certain of is an increase in litigation costs that would result from the proposal.

"Isn't it hard to imagine plaintiffs attorneys not trying to make hay with this? I don't see any scenario where plaintiffs' lawyers won't try to take private action out for a test drive, to the great expense of our industry," Carroll said.

For however accurate the DOL's cost estimates are, its clear from its analysis that broker-dealers would bear the brunt of the pain and expense in implementing the new rules.

The Best Interest Contract Exemption will result in 86 million new disclosures being distributed in the first year, and more than 66 million more each year after. Producing and distribution of the documents, which the DOL says could be up to 10 pages long, will cost the industry $77.4 million in the first year, and $29.2 million every year after, according to the DOL.

Principal Transaction Exemptions will result in millions more disclosure forms, and cost the industry more than $57 million in the first year, and almost $48 million every year after.

Benefitspro asked the DOL if it was fair to use SIFMA data as a base for its cost analysis, given that the data was provided to the SEC, for a wholly different rule.

The DOL also was asked if potential litigation costs should not be considered in a thorough analysis of the proposal.

"We stand by our economic impact analysis," a DOL representative said in a statement. "The public comment process is open and we welcome feedback on this and all parts of the proposal."