Oh even with a middle ground agency like UFA they make a great deal of money so can afford attractive salaries for young girls it's more than enough even when they are starting out. Most of it when they are young goes to supporting family and their rent etc if they needed to move to Tokyo i imagine so that is why they live on an allowance but still their spending money seems to be enough to live well. You get girls talking about going out for meals and stuff often and that's not exactly cheap in Tokyo.
But there are idols who are much lower down on the chain who struggle to make any real money and many idols chasing the dream drop out alltogether before they even got anywhere so it's not always guarenteed to make you a good living.
"Attractive" salaries? Considering that no one has backed out from a contract after being chosen in auditions, I don't think it has anything to do with attractive salaries when you join at first... mostly the dream and of course you went through all the tests so might as well be an idol high salary or not.