The reason usually quoted for importing eggs/day olds is cost. To fully equip a keeper from egg production through to release as poults is expensive,no arguments on that. How it's costed over so many seasons is for the boss and his financial wizard adviser/accountant.
However,as with all hobbies or personal interests, some people will spend the money.
As keeper here, I operate closed flock system for egg production. Incubation, hatching and rearing is all done here on the estate. We do enough here to supply the boss's other estate with poults for the part time keeper to the run on from 6/7 weeks.
The boss is obviously happy with the money aspects or I'd be unemployed! A lot depends on you, some people spend lots, some people don't.
There's no one price fit's all, whether that's shooting, stalking, fishing or other leisure pursuits. Think of what is spent on horse's,cars,boats etc. People spend what they're comfortable with.