I suppose there's two ways of looking at it. If you have a serious interest in the topic then subscribing to the magazine is a good way of learning and collecting information with the model as a bonus. If you subscribe just to build a model then it's a rip off.
Its OK if you really want to spend such huge amounts of cash on a model. Its OK if they keep producing it. You will probably find that after the first couple of issues you will have to get it by subscription, and if they don't get enough people continuing to buy it it could well end up being cancelled and you have a partly finished model.
If you really want to build a model a visit to a good retailer will get you something comparable at FAR less cost, and you will get all the parts at once not over a couple of years.
I remember a special edition of Antiques Roadshow a few years back, it was about modern collectables, and a guy turned up with a Star Trek 3D Chess Set he'd bought from the back of a colour supplement, took about 3 years to collect all the pieces at a four figure cost. The expert reckoned at a good auction he'd be lucky to recoup £100...
It's hard luck on a mother who has a child who pesters her to buy the first couple of parts of the model because it is practically free. After that the cost rockets per magazine and per part. If she hasn't really read the subject up beforehand she is then in a quandry as to what to do because she knows she can't afford it.
The short answer is, yes it is a rip off and a big one at that.