NEW YORK – Andy Cohen, 47, uses exchange-traded fund portfolios recommended by MarketRiders Inc. to manage his retirement plan and children’s college savings. He’s charged about $10 a month.

“I don’t have enough money that I have a dedicated wealth manager,” said Cohen, who lives in San Mateo, Calif., and is chief executive officer of Caring.com, which offers resources for children helping aging parents. “It got me a much more diversified portfolio than I could have ever done on my own.”

The next thundering herd on Wall Street may be the ranks of low-cost portfolio managers such as MarketRiders and Folio Investing, which cater to self-directed investors like Cohen. Sites that sell prepackaged portfolios have attracted more than $3 billion in assets over the last three years as more investors leave their full-service brokers.

“Individual investors have started to realize they can actually do some things as self-directed investors reasonably well, if they’re given a platform that allows them to invest more intelligently,” said Steven Wallman, chief executive officer of Folio Investing, where investors can purchase predesigned and customized index portfolios for $29 a month.

Some of the firms, such as Flat Fee Portfolios, are too new to have any performance history. MarketRiders can’t track the actual performance of its customers’ accounts, since it doesn’t have custody of their assets. Covestor and Wealthfront Inc., which give users access to third-party investors, publish performance history for the managers they work with on their sites.

“Who are the people that are advising me when I’m going to a faceless website?” said Chris Walters, head of wealth management for Pasadena, Calif.-based CitizensTrust. He said investors should be concerned by the lack of performance history available from some of the firms.

Traditional brokerages are focusing more on their wealthiest clients in an effort to improve profitability, so the customers leaving these firms tend to be the ones with the smallest accounts, said Katharine Wolf, senior analyst for Cerulli Associates, a Boston-based research firm.

“They’re not looking to target a client that has, say, under $250,000 to invest,” she said.

Those investors are potential customers for services such as Flat Fee Portfolios, which began opening accounts in February. Clients with assets of less than $250,000 are offered several predesigned portfolios with an annual review for a fee of $129 a month.