Costs have never really made sense.
Light Engines are cheaper than XLs, despite, at their introduction, certainly being more cutting edge tech.
ER Lasers cost ~twice as much as regulars, despite being regularly available for a good century.
An Engine Cost curve of 1/2.5/3/5 would probably be more fitting.
The costs also never factor in the realities of logistics, like Endosteel requiring Orbital Factories; That's probably a consideration that results in price if you have reason to believe they might be taken from you.
Anyways, different topic.
If the projected cost was correct, barely anyone would buy this mech.
If it's only the cost of two assault mechs, it's a good buy both for prestige reasons and to fill out elite formations, as your best pilots may get exponential results with the best mechs.