This might surprise you, but I don't agree. Those other companies that make metal models do limited runs of maybe a couple hundred, or a thousand at best. GW makes thousands of models. The more you make, the cheaper they become to produce. They have a state of the art plastics factory using CAD software, a well trained, specialized work force, and deep pockets.
There is no arguing the quality of the minis. In general, GW has one thing going for them: consistency. They produce more top quality models then most other companies. On the other hand, I have seen some fantastic mini's that blow the best GW has to offer out the window.
It's interesting that you bring up the metal discussion, as GW has now stopped using metal. Everything is being converted over to plastic/resin. It's amazing that they are able to still provide such great quality in this format! However, they are using a less expensive material and manufacturing process for that mini and raising prices.
This is a hobby, and like all hobbies, it's as expensive or cheap as your checkbook allows.