Renters beware. In a real estate forecast released this week, the USC Lusk Center said that while rents in the Inland Empire are the lowest in the Los Angeles Basin, they will rise the fastest over the next two years. Raphael Bostic, director of the center's Casden Real Estate Economics Forecast, said the current average rent in the Inland Empire is $ 850. That's a bargain compared to Los Angeles County, where the average rent is $ 1,200 and Orange County, where the average rent is $ 1,220. But by the end of 2004, Bostic said, the average rental rate in San Bernardino and Riverside counties is expected to increase about 15 percent. Apartment construction is picking up, said Bostic, but not enough to keep pace with burgeoning demand for apartments in a strong economic region that's a national leader in employment growth. "Although the Inland Empire accounts for 21 percent of the Los Angeles region's population, it has only 11 percent of the region's apartment stock, reflecting a preference for single-family homes," the forecast noted.