Sotheby's, which sold a Mark Rothko painting for more than $70 million in May, said it will raise fees by one-quarter for buyers of lower-priced art, following a similar move by Christie's International. Buyers of art priced at $20,000 or less will pay a commission of 25 percent, the company said in a filing with the U.S. Securities and Exchange Commission. Fees on big-ticket items remain at 20 percent of the first $500,000 in value, and 12 percent on the rest of the hammer price, it said. Sotheby's, which auctioned almost as much art as Christie's in the first half, has said its strategy is to focus on higher- priced works. The auction house sold 37,977 lots in the first six months, compared with 56,978 at Christie's, Sotheby's said in an e-mail. Christie's, which raised its fees on lower-priced lots early this month, wouldn't say how many lots it sold. Sotheby's price increase will be applied from Sept. 1 at all salerooms around the world, Sotheby's said.