Not once did Apple refer to the watch as a mini-phone, in fact, it's the opposite, it in no way replaces your phone, it augments it. It's a watch, it's just a modern, technologically rich watch with other capabilities.
Why are those other watches expensive for other reasons? There's four pricing things that come to mind, accuracy, which obviously this watch has, given it's sync'd to your phone and to the internet. Build quality, which this will certainly have. Design, this is subjective, but it's also flexible given the watch face is virtual and can be altered. Branding, Apple tax will certainly apply, as does Rolex, Omega, Breitling, IWC, etc.
It's not a mini-phone, it's just a feature rich watch that uses miniaturized computer technology to achieve that rather clockwork, or simpler digital timekeeping and displays.