By Daniel Kline Royal Caribbean, Carnival, and Norwegian cruise lines have faced a dark couple of years, but smoother seas (and higher prices) might be ahead. Cruise prices are a moving target. Cruise lines use a formula based on demand where prices change depending on how sailing is selling. Customers usually get the best deals by booking early, but that can vary if an individual sailing struggles to fill up. To further complicate matters, Royal Caribbean (RCL) – Get Royal Caribbean Group Report, Carnival Cruise Lines (CCL) – Get Carnival Corporation Report, and Norwegian Cruise Lines (NCLH) …