PHOTO: Kara Biggs recently bought a home with REX, which charges home sellers a 2 percent fee instead of the traditional 6 percent fee. REX can offer sellers that fee by declining to pay the buyer’s agent.

For the average Houston home, priced at $310,700, real estate agent commissions cost $18,600. If the same deal had closed in London, the homeseller would have only paid agents about $3,700.

The reasons for the difference are at the center of a class-action suit that could dramatically change the way homes are bought and sold in the United States.

The case, brought by a Minnesota homeowner, seeks to strike down the standard practice of agents splitting commissions. If successful, the suit would potentially save individual home sellers thousands of dollars in commissions, but it would also cut the earnings of real estate agents across the country — including some 37,000 in the Houston area — and put more pressure on traditional brokerages, already contending with a host of discount and online competitors.