Miami-based real estate developer Related Group and partner Alta Developers have signed a contract for a 54th-floor, 5,893-square-foot triplex penthouse at Casa Bella Residences by B&B Italia for $12.6 million, or $2,138 per square foot, signifying what will be the most expensive condominium sale on a price-per-square-foot basis in Downtown Miami’s highly desirable Arts and Cultural District. Casa Bella Residences by B&B Italia has achieved nearly $50 million in signed contracts over the past 60 days.