At 2:26 p.m. one day in April, an investor bought $30,000 of bonds issued to finance a new stadium for the New York Mets at 106 cents on the dollar. Six minutes earlier, a broker paid 101 cents for the same securities.

Chances are, if the buyer was one of the individual investors who dominate the $3.7 trillion municipal market, he or she was unaware of paying a $1,500 markup. That will change if regulators adopt a new rule requiring brokers to disclose their profits on trade confirmations.

“A year’s worth of interest can be wiped away simply by buying or selling a bond,” Securities and Exchange Commission commissioner Michael Piwowar said in a telephone interview. “People should know what they’re paying.”

The proposal by the Municipal Securities Rulemaking Board, which is soliciting comments on it through Dec. 11, follows a push by the SEC to inject more transparency into the market for state and city debt to protect mom and pop customers. Such buyers own about 42 percent of the securities, according to the Federal Reserve’s figures. That’s six times more than their share of the Treasury market.

By requiring brokers to disclose how much they earn, regulators want to foster more competitive pricing among dealers and drive down trading costs. A study by the Securities Litigation & Consulting Group, a firm that advises on lawsuits, estimated that customers paid more than $10 billion in excessive markups and markdowns on municipal bonds between 2005 and 2013.

Little Guys Pay

That’s mostly borne by small investors. Those who bought $100,000 or less of investment-grade muni debt in December 2013 paid brokers an average transaction cost of 1.73 percent, twice that of corporate bonds, according to a report by Standard & Poor’s.