Prices have declined in Miami, but none so much as Keystone Towers in North Miami, where unit prices are down 81 percent since peaking in 2007, according to data from CondoReports.com, a market data firm. The worst-hit complexes have had average unit price declines of 75 percent or more, the report said. At Keystone, an owner who bought a 1,000-square-foot unit for $334,000 would have seen a similar unit go for $62,000. The loss in value: $272,000. Adam Cappel, president of CondoReports.com, said only a fraction of 1 percent of the condos surveyed had increased in value from the market peak.