The "specification" is usually what the end user requires in terms of performance: how much current to carry, how much loss (iron and copper) is acceptable, applied voltage, method of protection, overall power quality, cycle of use (which may be continuous, but may not), environment of application, etc.
Note that above a certain rating, almost all (high power) reactors are custom products ... or at least semi-custom. That means the manufacture is done with present raw material prices - which are going to be higher than in recent years as the components of a reactor tend to be copper, steel, and insulating media.
Also - just because the cost seems to be a "fortune" to the OP does not mean it's a fortune to others. Quantitative description is (almost) always better than qualitative.