The flood of first-time homebuyers taking advantage of the government’s tax credit program has surely propped up sales activity, but it has also dragged down commissions for real estate agents and brokers closing deals. The year’s commission total dollar value — $40.6 billion — is at its lowest level since 2002, driven by the concentration of first-time homebuyers in the lower end of the market, Bloomberg reported. Even though sales were up and the average commission rate per transaction rose to 5.29 percent, the decline in average prices means that the take-home pay for brokers is significantly less, said Steve Murray, president of residential property research firm Real Trends. Nearly 75 percent of November home sales were in the $250,000 or less category, the National Association of Realtors said. “I had the busiest November I’ve had in five years, which made up for lower prices and lower commissions, but I know some people who left the business altogether or took second jobs because they were making so much less for each transaction,” said Karen McCormack, who owns a real estate brokerage in Boston.