Scattered around Los Angeles are some surprisingly valuable vacant lots disguised by weeds or broken blacktop or the remains of an unwanted building — and many have quietly come to market, thanks to the real estate collapse. Billions of dollars were lost by developers who bought land to build high-profile projects but weren’t able to get their plans off the ground, even after spending lavishly on architectural designs and other measures to get their buildings approved by local officials. As the real estate cycle plays out, the pained exit of ambitious builders has created an unusual abundance of opportunities to buy expensive eyesores. “There are a mess of these around town,” real estate appraiser Steven Norris said. “Dirty corners entitled for high-rises and they’re just parking cars on them.”