I suppose it's the sort of price that is typically seen as a good investment to get into a game, splitting the box with a friend, or painting it all yourself for demo games etc. Not sure if they are able to tell whether this is optimal based on their own sales numbers though (even GW doesn't exactly release enough similar products at slightly variable price points to tell what's best - indeed, they seem to simply have chosen this number and stuck with it for now), but it no doubt sells "well enough" if they continue using it.
Consistent pricing (if not in terms of the amount of product in a box, but simply the per-product price) also helps with customers' expectations.
Not sure how this one looks in terms of value for money - not cheap enough to start it with limited interest in the game, but at the same time not too expensive to be an obstacle if I was interested.