英语语法问题The car costs too much for us to buy it.The car is too

英语语法问题
The car costs too much for us to buy it.
The car is too expensive for us to buy.
The car isn't cheap enough for us to buy.
The car is so expensive that we canitbuy it.
有的加it有的不加,但是这四个都表示同一个意思.我想问一下,为何有的不加,有的加?
我想问的不是花费的问题，而是too……to句型、So……that句型和not ……enough句型后为什么有的加it，有的不加.